Let butter and eggs stand at room temperature for 30 minutes. Grease and flour two 9-inch round cake pans; set aside. In a medium bowl whisk together cocoa powder and boiling water until smooth. Let cool for 10 minutes. Whisk in buttermilk and vanilla; set aside. In medium bowl stir together flour, baking soda, and salt; set aside.

Preheat oven to 350F. In a mixing bowl beat butter with electric mixer on medium speed for 30 seconds. Add sugars; beat until combined. Add eggs, one at a time, beating until combined after each addition. Beat in one-third of the flour mixture on low speed just until combined after each addition. Beat in half of buttermilk mixture. Mixture may look curdled. Beat in half of the remaining flour mixture, then the remaining buttermilk mixture, and finally, the remaining flour mixture.

Divide batter between prepared pans, spread evenly. Bake about 30 minutes or until toothpick inserted near centers comes out clean. Cool in pans on wire rack 10 minutes. Remove cakes from pans. Let cool on wire rack. To assemble, place first cake layer on a serving plate. Spread 1-1/2 cups frosting over the top of the layer. Top frosted first layer with second cake layer, top side up. Frost top and sides of entire cake with remaining Chocolate Butter Frosting.

Chocolate Butter Frosting: In a very large mixing bowl, beat 3/4 cup butter, softened, and 1/2 cup unsweetened cocoa powder with an electric mixer on medium speed until smooth. Gradually beat in 2 cups powdered sugar until well combined. Gradually beat in 1/2 cup milk and 2 teaspoons vanilla until well combined. Gradually beat in 6 cups powdered sugar. If necessary, beat in additional milk, 1 teaspoon at a time, to reach spreading consistency.

Chocolate Cherry Cake: Prepare cake as directed above, except stir 1/2 cup coarsely chopped, well-drained maraschino cherries into the batter at the end of Step 2. To assemble, place one cake layer on a serving plate, top side up. Spread with 1 cup of the Chocolate Butter Frosting. Snip any large cherry pieces in 1/2 cup cherry preserves. Spread preserves on top of frosting on first cake layer, leaving a 1-inch space around the top edges of the cake. Top with second cake layer, top side up. Use the remaining Chocolate Butter Frosting to frost the top and sides of the whole cake. If desired, garnish the top of the cake with well-drained whole maraschino cherries.

Malted Milk Shake Cake: Prepare cake as directed above, except stir 1/3 cup malted milk powder with the flour, baking soda, and salt in Step 1. Prepare the Chocolate Butter Frosting as directed, except add 1/4 cup malted milk powder with the first 2 cups powdered sugar. If desired, garnish cake with 1 cup coarsely chopped malted milk balls.