Generously grease and flour two 9-inch round cake pans. Cream butter, shortening, sugar and vanilla until light and fluffy: blend in eggs.
Combine baking soda,cocoa. Pour, baking powder and salt in bowl: add alternately with milk to batter. Blend well. Pour into prepared pans; bake at 350 degrees for 30 to 35 minutes or until cake tester inserted in center comes out clean.
Cool 10 minutes; remove from pans.