In a bowl, combine the flour, sugar and 2 tablespoons cocoa. Stir in milk,oil & vanilla until smooth. Pour into a greased 8-in. square baking pan. Combine the brown sugar and remaining cocoa; sprinkle over batter. Slowly pour hot water over top (do not stir). Bake at 350° for 40-45 minutes or until top of cake springs back when lightly touched. Serve warm with ice cream if desired.