In a medium bowl, combine flour, white sugar, 2 tablespoons cocoa, baking powder, and salt. Stir in the milk, oil, and vanilla until smooth. Spread in ungreased 9 inch square baking pan.
Combine brown sugar and 4 tablespoons cocoa; sprinkle over batter in the pan. Pour hot water over all; do not stir.
Bake at 350 degrees F (175 degrees C) for 35 to 40 minutes. Serve warm.