In a small bowl, combine the baking mix, sugar and cocoa. Stir in milk and vanilla. Spoon into two greased 8- or 10-oz. custard cups.
For topping, combine the brown sugar and cocoa in a bowl. Stir in boiling water. Pour over batter. Bake at 350° for 25 minutes or until a toothpick inserted in the cake layer comes out clean. Top with ice cream or whipped cream if desired. Yield: 2 servings.