Cream butter and sugar until light and fluffy. Beat in eggs, one at a time. Sift together flour, baking soda, salt and cocoa; add alternately with buttermilk to creamed mixture. Stir in vanilla and warm water.
Pour batter into lightly greased and floured 13 X 9 X 2" pan. Bake in moderate oven (350°) about 45 minutes, or until the cake tests done.