This tender grapefruit poppy seed cake is bright and light. It is bursting with sweet and tart fresh grapefruit juice and is topped with a silky grapefruit Swiss meringue buttercream.

I have been sitting on this recipe for almost a year now. Mr. two sugar bugs is a gigantic grapefruit fan and asked me to create a cake highlighting this sweet and some what sour fruit. By the time this recipe was ready to share, grapefruit had gone out of season. I am so excited to share this cake today!

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I rarely have a recipe work the way I plan on the first try; this ruby red grapefruit poppy seed cake was no exception. It's harder to get the fresh grapefruit juice to shine through in a baked cake. So we might have had to eat multiple cakes in the process of building this recipe.😉😉
HOW DO YOU INFUSE THE FLAVOR INTO THE GRAPEFRUIT POPPY SEED CAKE?
This cake and buttercream recipe rely solely on fresh grapefruit for the flavor. You will use the zest and juice. If you need a good zester; this is the one I use from Microplane. I like it because it yields a very fine zest without any pith. It gets used for lemon bars, lemon cupcakes and strawberry lemonade cookies and it works like a charm every time!
In my initial test, the grapefruit flavor did not come through in the baked cake. Reducing the grapefruit juice on the stove solved that issue. You will also brush the warm cake with a juice and sugar syrup.
HOW TO GET MORE JUICE OUT OF THE GRAPEFRUIT
First, having the grapefruit at room temperature will help to yield more juice. Rolling the grapefruit back and forth on the counter with the palm of my hand a few times also helps. I honestly don't remember where I learned this trick, but it's an effective one!
Secondly, be sure to zest your grapefruit prior to juicing them. Using a mini strainer is also a great way to sift out any seeds that sneak into the juice!

IS HAVING ROOM TEMPERATURE INGREDIENTS REALLY IMPORTANT?
Yes! Room temperature ingredients will come together better, which will result in a lighter and better textured cake. Sally, at Sally's Baking Addiction, has a fantastic blog post covering all the science behind why this works. The light texture of this grapefruit cake makes a great contrast to the little crunch from the poppy seeds.

STEP BY STEP PHOTOS TO DECORATE THIS SHEET CAKE
One of my goals with this blog is to help you gain the confidence with decorating your baked desserts. Decorating layer cakes can be daunting and takes lots of practice. Sheet cakes, on the other hand, are much more approachable! I give you some ideas for decorating a sheet cake in this post, but I'll show you how to create this design too.
Start by filling a piping bag with your prepared grapefruit buttercream. You do not need a piping tip, just snip off the end of the piping bag with a pair of scissors. I used three colors, but this would look fabulous with a single color too.

Pipe a row of buttercream dots across the top short edge of the cake. Then take an off-set spatula and smoosh (a very technical term there!) and pull the frosting down an inch or so.

Continue making rows of buttercream dots and smooshing and pulling the buttercream down the cake until you have covered the whole cake.

I finished the decorating by scattering some Sweetapolita sprinkles over the buttercream.

Ta-da! A perfectly cute way to decorate your cake. Be sure to email me {tasia@twosugarbugs.com} a photo or tag @twosugarbugs on Instagram or Facebook when you use this technique. I cannot wait to see your creations!!
TIPS FOR MAKING THIS RUBY RED GRAPEFRUIT POPPY SEED CAKE
- Simmer the grapefruit juice over low heat for about 20-25 minutes. We aren’t going for a super thick reduction, but the goal is to intensify the grapefruit flavor. You will use this concentrated juice in the cake batter, the buttercream and to brush over the warm cake after baking.
- Whisk the poppy seeds in with the dry ingredients prior to adding them to the cake. This helps them incorporate better and lessens the risk of over-mixing your batter.
- Beat the grapefruit zest in with the butter and sugar. This helps to get the zest perfectly distributed throughout the cake.
- This cake can be made as a 9x13 sheet cake or as a three layer 6 inch cake.
- If you want more detailed tips for making Swiss meringue buttercream, see this post.
Ruby Red Grapefruit Poppy Seed Cake

This tender grapefruit cake is bright and light. It is bursting with sweet and tart fresh grapefruit and is topped with a silky grapefruit Swiss meringue buttercream. Can be made as a sheet cake or a three layer six inch cake.
Ingredients
- 2-3 Ruby Red Grapefruit, zest and juice
- 2 cups all purpose flour
- 2 teaspoons baking powder
- ½ teaspoon kosher salt
- 3 tablespoons poppy seeds
- ¾ cup unsalted butter, room temperature
- 1 ½ cups granulated sugar
- 1 tablespoon ruby red grapefruit zest
- 3 large eggs, room temperature
- 1 teaspoon pure vanilla extract
- ⅔ cup buttermilk, room temperature
- ¼ cup concentrated ruby red grapefruit juice
Ruby Red Grapefruit Swiss Meringue Buttercream
- 3 large egg whites
- ¼ cup concentrated ruby red grapefruit juice
- ¾ cup granulated sugar
- ½ teaspoon salt
- ¾ cup unsalted butter, room temperature*
- 1 tablespoon ruby red grapefruit zest
Instructions
- For the cake:
Zest the grapefruit and set the zest aside. Juice the grapefruit, straining out any seeds. 2-3 large grapefruit should yield about a ¾ - 1 cup of juice. Place the juice in a small saucepan and place over low heat; simmer for about 20-25 minutes. We aren’t going for a super thick reduction, but the goal is to intensify the grapefruit flavor. You will use ¼ cup juice in the cake and ¼ cup juice in the buttercream. The remaining grapefruit concentrate will be used as a soak after the cakes have baked. - Preheat your oven to 350° F.
- Prepare a 9x13 baking pan or three 6-inch round baking pans; butter the pans, line the bottom of each pan with parchment paper, butter again and dust lightly with flour. Set aside. With a 9x13 you can leave some parchment paper overhang.
- In a medium bowl, whisk together flour, baking powder, salt and poppy seeds. Set aside.
- In the bowl of a stand mixer fitted with the paddle attachment, cream butter, sugar and 1 tablespoon grapefruit zest on medium-high until light and fluffy, approximately 3-5 minutes. Scrape up and down the sides of the bowl and beat in the eggs one at a time. Then add the vanilla and beat on medium-high combined, about 2 minutes.
- In a measuring cup mix together the buttermilk and ¼ cup (reduced) grapefruit juice.
- With the mixer on low, add the flour mixture and buttermilk mixture in alternating additions, beginning and ending with the dry. I do flour, liquid, flour, liquid and flour. Be careful not to over mix your batter. The batter is slightly on the thicker side.
- Pour the batter into the prepared 9x13 baking pan or divide your batter equally between the three prepared pans and bake 25-30 minutes or until the cakes are baked through. To test, insert a toothpick or cake tester in the center of the cake; if it comes out mostly clean, its done.
- Take the remaining graperfruit concentrate (make sure you have saved the ¼ cup you need for the buttercream!) and mix with a few teaspoons of powdered sugar; then brush over the warm cake layers.
- Cool cake layers in pans placed on a wire rack for 10-15 minutes before turning them onto the wire rack to cool completely. If making a sheet cake, the cake can cool in the pan.
- For the grapefruit swiss meringue buttercream:
Wipe the bowl and whisk of your stand mixer with vinegar or lemon juice to make sure it is grease free or your egg whites may not stiffen. - Whisk egg whites, grapefruit juice, sugar and salt in the bowl of your stand mixer. Whisk and heat mixture over a saucepan of simmering water until hot to touch, sugar is fully dissolved (you should not feel any granules when rubbed between your fingers) and it’s a bit frothy, approximately 3-5 minutes.
- Remove the bowl from the saucepan and wipe the bottom dry. Place on your stand mixer and whisk on medium-high (8 on a Kitchen Aid stand mixer) until the mixture is cool and the meringue holds a stiff peak (the bottom of your bowl should also be cooled), approximately 10-20 minutes. Test to make sure the meringue is cool before proceeding. If it’s still a little warm, let it cool a few minutes before adding your butter.
- Switch to the paddle attachment, reduce the speed to medium (4 or 6 on a Kitchen Aid stand mixer), and add the butter, one tablespoon at a time, letting the butter fully incorporate before adding more. ** Once all butter is incorporated, scrape down sides of the bowl and add the grapefruit zest. Turn the mixer up to high and whip the buttercream for about 5 minutes.
- Decorate the completely cooled cake as desired.
Notes
* Your butter only needs about 1 hour of time (depending on temperature of your kitchen) to get to room temperature. You want the butter still slightly cool to the touch and not overly warm or soft.
** At some point your buttercream is going to be a sloppy mess. It may even look curdled. Do not fear, just keep whipping and it will magically come together into a beautiful whipped frosting. If you feel it is too soupy, place the mixture in the refrigerator for 20 minutes and then re-whip.
Buttercream yields enough to do a 9x13 sheet cake or a semi-naked layer cake with some decorative swirls. If you want a fully frosted layer cake and enough for also decorating, you will want to double the buttercream recipe.
Recommended Products
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KitchenAid KSM150PSER Artisan Tilt-Head Stand Mixer with Pouring Shield, 5-Quart, Empire Red
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PaperChef Parchment Paper for Baking: Multipurpose Nonstick Parchment Sheets, Oven Safe, No Grease or Butter Needed, Ideal for Cooking and Roasting, Biodegradable, Kosher-Certified (205 sq ft roll)
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USA Pan Bakeware Rectangular Cake Pan, 9 x 13 inch, Nonstick & Quick Release Coating, Made in the USA from Aluminized Steel
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Fat Daddio's PRD-62 Round Cake Pan, 6 x 2 Inch, Silver
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Microplane 38004 Profesional Series Fine Grater, 18/8, Stainless Steel
Nutrition Information:
Yield:
20Serving Size:
1Amount Per Serving: Calories: 302Total Fat: 15gSaturated Fat: 9gTrans Fat: 0gUnsaturated Fat: 5gCholesterol: 65mgSodium: 199mgCarbohydrates: 39gFiber: 1gSugar: 27gProtein: 4g
The nutrition information is an estimate and may not be entirely accurate.
Kim
I love anything grapefruit and this cake sounds perfect for me!! Thanks for all the great tips in here!
Tasia
Thank you Kim! I love how light and bright this cake is. It's perfect for winter and celebrating citrus season.
Katherine | Love In My Oven
I love grapefruit too!!! This is so beautiful! Those colors! You may have been sitting on it for one year, but I am pretty grateful that you finally shared it with the world. Yum! I'm so excited for citrus season. Hope you have a terrific New Year's, Tasia!
Tasia
Yay for grapefruit! Citrus season is definitely something to look forward to in the winter. So happy to have met you Katherine and wishing you a fabulous 2020!! xoxo