In medium bowl, combine flour, 3/4 cup granulated sugar, 2 tablespoons cocoa, baking powder and salt. Blend in milk, melted butter and vanilla. Beat until smooth. Pour batter into a 8" or 9" square baking pan.
In small bowl, combine 1/2 cup granulated sugar, 1 1/2 cups brown sugar and 2 tablespoons cocoa. Sprinkle mixture evenly over the batter. Pour the hot water over top; do not stir.
Bake 35 to 40 minutes in a 350°F oven or until center is almost set. Let stand 15 minutes. Spoon into dessert dishes, spooning sauce from bottom of pan over top.