In a medium bowl, combine flour, sugar, 2 tbsp. Cocoa, baking powder and salt. Stir in the milk, oil and vanilla until smooth. Spread in an ungreased 9 inch square baking pan. Combine brown sugar and remaining cocoa; sprinkle over batter. Pour hot water over all; do not stir. Bake at 350 degrees for 35-40 minutes. Spoon into bowls and serve warm. Top with whipped cream or ice cream.