In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in vanilla. Combine the flour, cocoa, baking soda, baking powder and salt; add to the creamed mixture alternately with water, beating well after each addition.
Pour into three greased and floured 9-in. round baking pans. Bake at 350° for 24-28 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pans to wire racks to cool completely. Fill and frost as desired.