In medium bowl, combine flour, sugar, 2T. cocoa baking powder and salt. Stir in the milk, oil and vanilla until smooth. Spread in an ungreased 9 in. square baking pan. Combine brown sugar and the remaining cocoa. sprinkle over batter. Pour hot water over all: do not stir. Bake at 350 for 35-40 minutes. Serve warm. Top with whipped cream or ice cream if desired.