In a medium mixing bowl, combine flour, sugar, 2 tablespoons cocoa, and salt. Stir in the milk, oil and vanilla until smooth. Spread in an un-greased 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. Serve warm. Top with whipped cream or ice cream if desired.