In a gallon sized ziplock bag, combine the two boxes of cake mix. Shake the bag to mix thoroughly.Lightly coat the inside of a microwaveable mug with non-stick spray. (Without the spray the cake will not transfer out of the mug.)(Here comes the 3-2-1 part) Into the prepared mug, stir together 3 tablespoons of cake mix and 2 tablespoons of water. Push three chocolate squares, one on top of the other, into the center of the batter. The cake will rise around the chocolate pile, so don’t worry if some of the chocolate is not submerged.Microwave on high for 1 minute.The cake can be eaten right out of the mug. If you want to transfer the cake onto a plate, run a knife around the edge between the cake and the mug to loosen, then dump out onto a plate.