Heat oven to 350 degrees. In medium bowl, combine 3/4 cup granulated sugar, flour, 3 tbsp cocoa, baking powder and salt. Blend in milk, melted butter and vanilla. Beat until smooth. Pour batter into a 8 or 9" square baking pan. In small bowl, combine 1/2 cup granulated sugar, 1 1/2 cups brown sugar and 4 tbsp cocoa. Sprinkle mixture, evenly, over the batter. Pour the hot water over top; do not stir. Bake 35 to 40 minutes in a 350 degree oven or until center is almost set. Let stand 15 minutes. Spoon into dessert dishes, spooning sauce from bottom of pan over top. Garnish with whipped topping.