Place butter in a 9 X 13-inch baking pan. Place in 350 F oven for about 5 minutes to melt; remove from oven and set aside. Combine in a bowl 1 1/2 c sugar, flour, milk and vanilla; stir to mix and pour over butter in pan. Combine 1 cup of sugar and cocoa in a small bowl and sprinkle over top of batter. Pour hot water over top of layered ingredients. Bake in 400 F oven for 30-35 minutes. Serve warm with ice cream if desired. Yield: 6-8 servings.