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 extract; 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 49 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.