To make 1 mug cake, spray microwavable mug (about 12 oz) with cooking spray. Place 3 tablespoons dry cake mixture into mug. Add 2 tablespoons water; stir well to blend. Place 3 chocolate bar rectangles (not overlapping) on top of cake batter in mug. Microwave uncovered on High 45 seconds to 1 minute or until cake is slightly moist on top. Do not overcook, or cake will be dry. Let stand 1 minute. Turn upside down onto serving plate, or eat out of mug. Top with whipped topping, if desired.