Mix together flour, salt, 2 tablespoons cocoa, baking powder and 1/2 cup granulated sugar in 2-quart deep baking dish.
Combine milk, melted butter and vanilla. Mix into dry ingredients in baking dish, beating well by hand.
Combine remaining 3 tablespoons cocoa, remaining 1/2 cup granulated sugar and brown sugar and sprinkle over cake batter. Pour hot water over all and place pan in oven.