12tablespoons(170 grams) unsalted butter, at cool room temperature
1 3/4cups(350 grams) granulated sugar
2large eggs, at cool room temperature and separated
1tablespoonvanilla extract
1cupwhole milk
For the milk chocolate buttercream:
2sticks (227 grams) unsalted butter, at room temperature
1large very fresh egg yolk, at room temperature (*3)
1teaspoonpure vanilla extract
1 1/4cups(156 grams) powdered sugar,sifted
1/4teaspoonfine salt
6ounces(170 grams) milk chocolate, melted and cooled
Instructions
Make the cake:
Preheat the oven to 350°F. Butter, flour, and line two 8-inch cake pans with parchment circles.
In a medium bowl whisk together the flour, baking powder, and salt.
In a large bowl with an electric mixer, beat the butter on high speed until smooth and creamy, about 1 minute. Add the sugar and beat on high speed for 3 to 4 minutes, until light and fluffy. Add two egg yolks and the vanilla and beat on medium-high speed until combined.
With the mixer on low speed, add the flour mixture in three additions, alternating with the whole milk, beginning and ending with the flour, mixing just until combined. Do not overmix.
In a separate clean bowl with an electric mixer fitted with a whisk attachment, beat the 2 egg whites until they hold soft peaks, about 3 minutes. Gently fold into the cake batter. Do not overmix.
Divide the batter evenly among the prepared cake pans and bake for about 30 minutes, or until a toothpick comes out clean. Let cool for 20 minutes in pans before turning out onto a cooling rack. For easier frosting, freeze the cake layers until firm.
Make the buttercream:
In a large bowl with an electric mixer, beat the butter on medium-high speed until light yellow and fluffy, about 3 minutes. Add the egg yolk and vanilla and continue beating for 3 minutes. Turn the mixer to low and gradually add the powdered sugar then salt. Increase speed to medium and beat until smooth and creamy.
On low speed, add the melted cooled chocolate and mix until just combined. Don't whip! Fill and spread immediately on the frozen cake. Serve or let stand covered at room temperature for up to 8 hours.
Video
Notes
Be sure to measure the flour correctly using a kitchen scale or spoon & level. Too much flour will make for a dry / dense cake. For best results, use bleached flour.
If you live at a high altitude, you'll likely need to reduce the baking powder by half or more.
Use a very fresh egg yolk. If worried about food safety, use a pasteurized shell egg or omit the egg yolk completely.