In a medium bowl combine flour, sugar, 2 tablespoons cocoa, baking powder, and salt. Stir in the milk and vanilla. Spread in an ungreased 9-inch square pan. Combine brown sugar, remaining 4 tablespoons cocoa, and water. Carefully pour hot water mixture over batter. DO NOT stir into batter. Bake at 350 for 35 to 40 minutes. Serve warm.
Great with ice cream too.