In a medium bowl, combine flour, sugar, 2 tablespoons cocoa, baking powder, and salt.
Stir in the milk, oil, and vanilla until smooth.
Spread in an ungreased 9" square baking dish.
Combine brown sugar and remaining cocoa; sprinkle over batter.
Pour hot water over it all. Do NOT stir.
Bake at 350 degrees F for 35-40 minutes. Serve warm.