Preheat oven to 350 degrees. In bowl, stir together 3/4 cup sugar, flour, 3 tablespoons cocoa, baking powder and salt. Stir in milk, butter and vanilla. Beat until smooth. Pour batter into an ungreased 8 or 9 inch square baking pan. Stir together remaining sugar, brown sugar and remaining cocoa; sprinkle mixture evenly over batter. Pour hot water over top; do not stir. Bake for 35 to 40 minutes 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.