Yellow Cake with Chocolate Buttercream

Yellow Cake:

12 tablespoons unsalted butter, at room temperature, plus extra for the pans

1 3/4 cup sugar, divided

1 1/2 teaspoons vanilla extract

7 large egg yolks

3 cups cake flour

1 teaspoon kosher salt

3/4 teaspoon baking soda

1 cup plus 2 tablespoons sour cream

7 large egg whites

Butter the bottoms and sides of three 8 or 9-inch cake pans. Line the bottom of the pans with parchment paper and then butter the top of the paper. Dust each pan with all-purpose flour and tap out the excess.

Preheat the oven to 350 degrees.

In the bowl of a stand mixer, fitted with the paddle attachment, mix the butter, 1 1/2 cups sugar, and the vanilla extract on medium speed, scraping down the sides of the bowl several times until pale and fluffy, about 5 minutes.   Add the egg yolks one at a time, letting each become incorporated into the batter before adding the next.

In a medium bowl, sift together the flour, salt, and baking soda. Reduce the mixer to low speed and alternate adding the flour mixture and the sour cream to the butter mixture, adding one third of each at a time and letting them fully mix in before adding the next. When all of the flour mixture, and the sour cream have been added, turn the mixer to medium-high speed and mix for about 2 minutes to fully incorporate the ingredients. 

Add the egg whites to a large, glass mixing bowl. Using a handheld mixer, whip until the egg whites are frothy. While mixing, slowly pour in the remaining 1/4 cup of sugar, and mix until the egg whites have soft peaks. Fold the egg whites into the cake batter in 3 additions. 

Divide the batter between the three prepared pans and place the pans on baking sheets. Bake the cakes, rotating the pans halfway through until they are lightly golden on top, gently bounce back when touched, and a toothpick inserted in the center of the cake comes out clean, about 25-30 minutes.

Let the cakes cool in the pan for a half hour before turning out to a wire rack to cool completely. Wait until the cakes have cooled completely before frosting.

Chocolate Frosting:

1 1/2 cups unsalted butter, at room temperature

2 tablespoons milk

9 ounces semi sweet chocolate, roughly chopped

1 teaspoon vanilla extract

2 1/4 cups confectioner's sugar, sifted

Melt the chocolate in a glass bowl placed over a saucepan of simmering water, stirring occasionally until no pieces of chocolate remain. Remove from the heat, and set aside to cool.

In a stand mixer, fitted with the paddle attachment, beat the butter until light and fluffy, about 3 minutes.  Carefully add the milk and beat until smooth.  Add the melted chocolate and beat for another 2 minutes. Add the vanilla and beat for another 3 minutes. Turn the mixer to low speed and gradually add the confectioner's sugar until the mixture is creamy and of the desired consistency.

To ice the cake, use a serrated knife to cut the tops of each layer so that they are even. Place one layer on a cake plate and spread the top with 3/4 cup of buttercream. Repeat with the second layer. Place the third layer on top. Using an offset spatula, spread the remaining buttercream over the entire cake.