Heat oven to 350°F. Combine 3/4 cup granulated sugar, flour, 1/4 cup cocoa, baking powder and salt. Stir in milk, butter and vanilla; beat until smooth.
Pour batter into ungreased 9-inch square baking pan. Stir together remaining 1/2 cup granulated sugar, brown sugar and remaining 1/4 cup cocoa; sprinkle mixture evenly over batter. Pour hot water over top; do not stir.
Bake 35 to 40 minutes or until center is almost set. Remove from oven; let stand 15 minutes.
Serve in dessert dishes, spooning sauce from bottom of pan over top. Garnish with whipped topping.
About 8 servings.