Preheat oven to 350°.
Lightly spoon flour into a dry measuring cup, and level with a knife. Combine the first 5 ingredients in a medium bowl. Stir in milk and vanilla extract. Spoon the batter into a 9-inch square baking pan. Combine brown sugar and 3 tablespoons cocoa, and sprinkle over cake batter. Pour water over batter (do not stir). Bake at 350° for 40 minutes or until cake springs back when lightly touched in center. Serve warm.