In a large bowl, cream butter and sugars. Add eggs, milk and vanilla; beat well. Combine flour, cocoa, salt, baking powder and soda; add to creamed mixture and mix well. Stir in water; mix until smooth. (Batter will be thin.) Pour into a greased and floured 13-in. x 9-in. baking pan. Bake at 350° for 35-40 minutes or until cake tests done. Cool on a wire rack. Dust with confectioners' sugar if desired.