Heat oven to 350°F. Combine ¾ cup granulated sugar, flour, ¼ cup cocoa, baking powder and salt. Stir in milk, butter and vanilla; beat until smooth.
Spread batter in ungreased 9-inch square baking pan. Stir together remaining ½ cup granulated sugar, brown sugar and remaining ¼ 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, if desired.
