Preparation

Bake the cake

• Pour half of the milk into a medium saucepan, and add the white chocolate. Warm over medium heat, whisking constantly, until the white chocolate has fully melted. Pour into a large glass measuring cup, stir in the remaining milk, followed by the vanilla, and then refrigerate until no warmer than 70°F, about 1 hour. (If you’re in a hurry, this process can be sped up with an ice bath; if not, it can be done up to a week in advance, then brought to 70°F at room temperature before use.)
• Line two 8-inch aluminum cake pans with parchment, and coat the pans and parchment lightly with cooking spray. Position a rack in the center of the oven, and heat the oven to 350°F.
• Sift the flours and baking powder into a medium bowl, and whisk thoroughly for up to a minute to fully combine.
• In the bowl of a stand mixer fitted with the whisk attachment, combine the sugar, salt, nutmeg, and egg yolks. Mix on low speed to moisten, then increase the speed to high. Whip until extremely pale and thick, and roughly doubled in volume, about 3 minutes.
• Reduce the speed to medium low, and drizzle in the oil, followed by the white chocolate-milk mixture, pouring each in a steady stream. Whisk in the flour mixture gently by hand to form a thin, lump-free batter, then fold once or twice with a flexible spatula to ensure that the batter is well mixed from the bottom up. Divide evenly between the prepared pans, about 26 oz. (about 3-3/4 cups) of batter per pan.
• Bake until well risen, golden brown, and firm to the touch, about 40 minutes. (The cakes should seem springy and firm when tapped with two fingers, although the cake visible through the cracks may seem slightly puffy and soft. Cool the cakes to room temperature in the pans, then cover until cool enough to frost (up to six hours).

Make the buttercream

Fill a medium saucepan with 1 or 2 inches of water, and bring to a boil over high heat.

Meanwhile, combine the egg whites with the sugars, salt, and cream of tartar in a large glass or ceramic bowl, stirring with a flexible spatula to combine. Place over the pan of water, and adjust the heat to maintain a gentle simmer.

把蛋清,搅拌和刮弓l continuously with the spatula, until warmed to 165°F. Remove from the heat, and transfer to the bowl of a stand mixer fitted with the whisk attachment. Whip on high speed until the meringue is thick, glossy, and cooled to about 90°F.

Reduce the speed to medium high, and add the butter 2 Tbs. at a time, one addition right after the next, followed by the vanilla. Stop the mixer, and pour in the melted white chocolate, then immediately begin mixing at medium speed. When the white chocolate seems mostly incorporated, scrape the bowl and whisk, then rewhip on high for a few seconds until smooth.

The buttercream should be glossy and hold stiff peaks. (Properly made, a cup of buttercream will weigh about 6 oz., with a temperature around 72°F.) Use right away, or transfer to a gallon-size zip-top bag and freeze until needed (thaw to about 72°F and rewhip before use).

Assemble and decorate the cake

• Carefully remove the cake layers from the pans by inverting onto racks, then flipping them top side up onto another rack. Level the layers with a serrated knife, and place one on a large, flat serving plate, cut side up. Cover with 1-1/2 cups buttercream, using an offset spatula to spread it evenly from edge to edge. Top with the second layer, cut side down, and cover with a 1-1/2-cup layer of buttercream. Spread another cup of buttercream around the side of the cake in a thin, even layer, then refrigerate the cake, uncovered, until the buttercream is cold and firm, about 35 minutes.
• Rewhip the remaining frosting, and generously coat the chilled cake in a thick layer of buttercream. Use the back of a spoon to create old-fashioned swoops and swirls of frosting, or whatever design you prefer.
• Top the cake with tart red fruits like fresh raspberries or cherries, if you like. (Before serving, set the cake out at room temperature until warmed to an internal temperature of about 70°F, as the buttercream can seem greasy when cold.) Store at room temperature until ready to serve, up to 24 hours.