Yellow Cake with Chocolate Buttercream

What can I say? I am a fool for decorating cakes. Someone wanted a cake and I designed it for them. This chocolate frosting is so good. Can't beat yellow cake with chocolate frosting.
YELLOW BIRTHDAY CAKE
3/4 cup butter, at room temperature
2 cups sugar
4 large eggs, separated
2 tsp vanilla extract
2 3/4 cups all purpose flour
1 1/2 tsp baking powder
1/2 tsp baking soda
1/4 tsp salt
1 cup buttermilk

1. Preheat oven to 350 degrees. Grease and flour pan ( or use one of those baking sprays that has flour in it)
2. Cream butter for 1-2 minutes; add the sugar and cream for another 2 minutes. Add the egg yolks and beat for another minute or until mixture appears light and fluffy. Beat in vanilla extract
3. In a separate bowl mix flour, baking powder, baking soda and salt. Add a third of the flour mixture to butter-sugar mixturea nd mix well. Then mixing,(but not overbeating) affter each addition, add half the buttermilk, followed by another third of flour, remaining buttermilk, and finally the last of the flour.
4. Beat the egg whites until stiff. Gently fold into batter with a rubber spatula. Pour into prepared pan and bake until a toothpick comes out clean.

2- 8 or 9 inch pans 45 minutes
13 by 9 by 2 inch 45 minutes
24 cupcake tins 20 minutes

Chocolate Buttercream Frosting
1 cup unsalted butter, at room temperature
2 1/2 cups sifted powdered sugar

1 cup cocoa
1 tsp vanilla extract
2 to 4 T milk

With an electric mixer, beat butter, sugar, and vanilla at low speed. Add milk bit by bit until mixture reaches a spreadable consistency.