For cake layers
Cake and frosting adapted from Baked: New Frontiers in Baking
Yield: Three 7- or 8-inch cake layers
2 1/2 cups of cake flour
3/4 cup all-purpose flour
1 tablespoon baking powder
1 teaspoon baking soda
3/4 teaspoon salt
1/2 cup (8 tablespoons) unsalted butter, softened
1/2 cup vegetable shortening
1 3/4 cups sugar
1 tablespoon pure vanilla extract
1 large egg
1 1/2 cups ice cold water
3 large egg whites, at room temperature
1/4 teaspoon cream of tartar
1. Preheat the over the 325 degrees F. Butter and flour three 7- or 8-inch round cake pans with removable bottoms. Sift the flours, baking powder, baking soda, and salt together in a large bowl. Set aside.
2. In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and shortening on medium speed until creamy, three to four minutes. Add the sugar, and beat on medium speed until fluffy, about three minutes. Scrape down the bowl, add the vanilla and whole egg, and beat until just combined. Turn the mixer to low. Add the flour mixture, alternating with the ice water, in three separate additions, beginning and ending with the flour mixture. Scrape down the bowl, then mix on low speed for a few more seconds.
3. In a medium bowl, whisk the egg whites and cream of tartar until soft peaks form. Do not overbeat. Gently fold the egg whites into the batter. Divide the batter among the prepared pans and smooth the tops. Bake for 40 to 45 minutes, rotating the pans halfway through, until a toothpick inserted in the center of the cake comes out clean. Transfer the cakes to a wire rack and let cool for ten minutes, then carefully remove cakes from pans and let cool completely. Remove the parchment.
4. A tip for assembly: Have frosting and filling prepared when ready to assemble cake. The most important part of the whole she-bang? Pipe a generous, tall border of frosting around the edges of each layer, and fill the well with about a half-cup of grapefruit cream. The border will keep the cream from sandwiching out between the layers.
For white chocolate ermine frosting
6 ounces white chocolate, coarsely chopped
1 1/2 cups sugar
1/3 cup all-purpose flour
1 1/2 cups milk
1/3 cup heavy cream
1 1/2 cups (3 sticks) unsalted butter, soft but cool, cut into small pieces
1 teaspoon pure vanilla extract
1. Using either a double boiler or a microwave, melt the white chocolate and set it aside to cool.
2. In a medium heavy-bottom saucepan, whisk the sugar and flour together. Add the milk and cream to cook over high heat, whisking constantly, until the mixture comes to a boil. Allow to boil for one minute, whisking quickly the whole time and being very careful not to scorch; all told this takes about ten minutes.
3. Transfer the mixture to the bowl of an electric mixer fitted with the paddle attachment. Beat on high speed until cool; the goal is to have the mixture be cool enough to emulsify rather than melt the butter. Reduce the speed to medium and add the butter; mix until thoroughly incorporated. Increase the speed to medium-high and beat until the frosting is light and fluffy.
Add the vanilla and white chocolate and continue mixing until combined; it should thicken immediately. Use right away or, if frosting is too thin, thicken it up in the fridge for 20 to 30 minutes until it reaches desired consistency.
For the grapefruit cream filling
Adapted from Tartine Bakery, via Food52
Yield: 2 ½ cups, about twice what you'll need for a cake
1 cup grapefruit juice, reduced to ½ cup
2 tablespoons lemon juice
3 large eggs
1 large egg yolk
6 tablespoons granulated sugar
12 tablespoons unsalted butter, cut into tablespoon pieces
1. Pour water to a depth of about 2 inches into a saucepan, place over medium heat, and bring to a simmer. Combine the juice, whole eggs, yolk, sugar, and salt in a stainless steel bowl that will rest securely in the rim of a saucepan over, not touching, the water. (Never let the egg yolks and sugar sit together for more than a moment without stirring; the sugar will cure the yolks and turn them granular.)
2. Place the bowl over the saucepan and continue to whisk until the mixture becomes very thick and registers 180 degrees F on a thermometer—ten to 12 minutes. Remove the bowl from over the water and let cool to 140 degrees F, stirring from time to time to release the heat.
3. When the base is cool, pour it into a countertop blender. With the blender running, add the butter one tablespoon at a time, blending after each addition until incorporated before adding the next piece. The cream will be pale yellow and thick. It can be used immediately, but I had better results letting it thicken up in the fridge more. The cream will keep covered in the fridge for five days; do not store in a metal bowl.