Preheat oven to 350°. In a large bowl, whisk flour, sugar, 2 tablespoons cocoa, baking powder and salt. In another bowl, whisk milk, oil and vanilla until blended. Add to flour mixture; stir just until moistened., Transfer to an ungreased 9-in. square baking pan. In a small bowl, mix brown sugar and remaining cocoa; sprinkle over batter. Pour hot water over all; do not stir. , Bake 35-40 minutes. Serve warm. If desired, top with ice cream.