1 1/2 sticks (12 tablespoons) unsalted butter, at room temperature,
2 1/2 cups cake flour
1 1/2 teaspoons baking powder
1/2 teaspoon baking soda
1/4 teaspoon salt
1 1/4 cups granulated sugar
2 large eggs plus 2 large egg whites
1 teaspoon pure vanilla extract
1 cup buttermilk
Dark Chocolate Frosting
1/4 cup plus 2 tablespoons unsweetened Dutch-process cocoa powder
1/4 cup plus 2 tablespoons boiling water
1 1/2 cups (3 sticks) unsalted butter, room temperature
1/2 cup confectioners' sugar, sifted
1/2 teaspoon salt
1 pound of good quality semisweet chocolate, melted and cooled
For the Vanilla Cake
Spray two 8- inch round baking pans with non-stick baking spray
Position an oven rack in the middle of the oven and preheat to 350 degrees F.
Sift together the flour, baking powder, baking soda and salt in a bowl. Set aside. Cream the butter and granulated sugar together in a mixer fitted with the paddle attachment on high speed until light and fluffy, about 2 minutes. Beat in the eggs one at a time, then the egg whites. Beat until smooth, about 30 seconds. Beat in the vanilla.
Reduce the speed to medium and add the flour mixture in 3 parts, alternating with the buttermilk and beginning and ending with the flour, making sure to scrape the sides of the bowl several times. Beat on high to smooth the batter, 10 to 15 seconds. Remove the bowl from the mixer and fold the sprinkles in by hand.
Divide the batter between the 2 pans and smooth the tops with a spatula. Bake until a tester inserted in the centers comes out clean, 30 to 35 minutes. Cool the cakes on a rack for about 15 minutes, then remove from the pans, cool completely.
For the Dark Chocolate Frosting
Combine cocoa and the boiling water, stirring until cocoa has dissolved. With an electric mixer on medium-high speed, beat butter, confectioners' sugar, and salt until pale and fluffy. Reduce speed to low. Add melted and cooled chocolate, beating until combined and scraping down sides of bowl as needed. Beat in the cocoa mixture. If not using immediately, frosting can be refrigerated up to 5 days, or frozen up to 1 month, in an airtight container. Before using, bring to room temperature, and beat on low speed until smooth again.
Set one cake layer on a serving platter or cake turntable if you have one. Spread the top with about 1 1/2 cups of the frosting to the edges. Top with the other cake layer, flat-side up. Spread the top of the cake with a large cup of frosting and smooth with a large offset spatula. Spread the remaining frosting on the sides of the cake. Hold the offset spatula at a 45-degree angle to the cake and rotate the cake (or turntable) to smooth the edges.
Allow cake to set it’s layers for a minimum of 30 minutes before serving but an hour is best.