Yield: One 3 to 4-layer, 8-inch cake

For the champagne cake:

• 3 cups (360 g) all-purpose flour, sifted
• 3 tsp baking powder
• 1 tsp salt
• 1 cup (2 sticks) unsalted butter, room temperature
• 2 cups (400 g) sugar
• 6 large egg whites, room temperature
• 2 tsp vanilla extract
• 1  1/2 cups champagne

For the grapefruit curd filling:

• 6 large egg yolks, room temperature
• zest of 1 grapefruit
• 2 cups grapefruit juice (about 4 grapefruits)
• 1  1/4 cups sugar (250 g)
• 1/4 tsp salt
• 1/4 cup cornstarch
• 3 tbs unsalted butter, chilled

For the champagne buttercream frosting:

• 1/2 cup + 2 tbs large egg whites, room temperature (about 6 large eggs)
• 1 cup (200 g) sugar
• 2 cups (4 sticks) unsalted butter, room temperature
• 2 tsp vanilla extract
• 1/2 cup champagne

Other:

• 8-inch cake pan(s)

Make the champagne cake:
Preheat 350°F. Grease and line four 8-inch cake pans (or do two batches).

Whisk together the flour, baking powder, and salt and set aside.

Cream the butter and sugar with the paddle attachment of your mixer on medium speed until it is light and fluffy, about 3 to 5 minutes. Scrape down the bowl.

With the mixer on medium-low, add in the egg whites one at a time, and then the vanilla. Mix until incorporated.

Now alternate adding the flour and champagne, beginning and ending with the flour mixture. You should have 3 additions of the flour mixture and two additions of the champagne.

Distribute a quarter of the batter into each cake pan. Smooth the tops with a spatula and slam it on the counter a few times to eliminate air bubbles. Bake for 15-25 minutes, or until a cake tester comes out clean.

Let it cool in the pans on a wire rack for 10 minutes before turning it out onto the rack to cool the rest of the way.

Make the grapefruit curd filling:

In a small bowl, whisk together the egg yolks and grapefruit zest. Set aside.

In a saucepan, combine the sugar, cornstarch, salt, and grapefruit juice. Whisk until it’s combined and place it over medium heat. Let it come to a boil, whisking intermittently. Once it comes to a boil, whisk continually for two minutes to avoid burning the curd.

Take it off the heat and add about 1/2 cup of the hot curd into the egg yolk mixture, whisking the yolks constantly as you add it in to avoid scrambling the eggs. Once the yolks have tempered with the curd, dump all of the yolks into the saucepan and return it to the heat.

Return the mixture to a boil and cook for about 5 minutes so it can thicken, whisking constantly to keep it from boiling or cooking unevenly.

Take it off the heat and whisk in the cold butter until it is thoroughly combined. Pour this into a shallow dish, cover with plastic wrap, and let it cool in the refrigerator for at least 2 hours before using.

NOTE: This will make about 2 cups of curd, and you won’t be using all of it in this cake. Looks like you’ll have to save some for yourself, oh darn.

Make the champagne buttercream:

Whisk the egg whites and sugar in the bowl of a stand mixer bowl. Place the bowl over the pan of simmering water and heat the egg whites until they are hot to the touch or 160°F. Return the bowl to the electric mixer and whisk on high speed for 10-15 minutes, or until the meringue is cool and holds a medium-stiff peak.

Swap the whisk attachment to a paddle attachment and with the mixer on low, add the butter one tablespoon at a time. Then add in the vanilla and beat it on medium-high until the frosting is silky and fluffy.

With the mixer still running, slowly pour in the champagne and beat until thoroughly combined.

Assemble the cake:

Once the cakes and curd are cooled, it’s assembly time! Place a cake layer on the bottom and spread a layer of curd (about 1/4 cup) and then a heap of buttercream on it. Top it with another cake layer, and spread another layer of curd and buttercream on it. Top with one final cake layer. Save the last extra cake layer for cake emergency or for snacking.

Do a crumb coat of the buttercream around the entire outside of the cake. Pop this into the refrigerator for 30 minutes to allow the cake and filling to set up.

Once the cake and filling are set, take it out of the fridge and frost the entire outside of the cake with the rest of the buttercream. Store in a cool area for at least an hour before serving to let everything set together (I usually have it set overnight on the countertop).