Combine 3/4 cup sugar, flour, 1/4 cup cocoa, baking powder and salt. Stir to combine.
Add in milk, butter and vanilla; beat until smooth.
Spread batter in 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 but do not stir.
Bake 35 to 40 minutes or until center is almost set. Remove from oven and let stand 15 minutes before serving. Serve in dessert dishes, spooning sauce from bottom of pan over top.