CAKE: Preheat oven to 350°F. Grease and flour two 9 inch cake pans. In a medium bowl, stir together the sugar, flour, cocoa, baking soda, baking powder and salt. Add eggs, milk, oil and vanilla Mix for 3 minutes with an electric mixer. Stir in the boiling water by hand. Pour evenly into the two prepared pans. Bake for 30 to 35 minutes, until a toothpick inserted comes out clean. Cool for 10 minutes before removing from pans to cool completely. FROSTING: Cream butter until light and fluffy. Stir in cocoa and confectioners' sugar alternately with the milk and vanilla. Beat to a spreading consistency. TO FROST: Split the layers of cooled cake horizontally. Cover the top of each layer with frosting, then stack them onto a serving plate. Frost the outside of the cake.