In a gallon sized ziplock bag, combine the two boxes of cake mix. Shake the bag to mix thoroughly.
If using a microwaveable mug, lightly coat the inside with non-stick spray (without the spray the cake will not transfer out of the mug.) or use a microwave safe paper cup.
(Here comes the 3-2-1 part) Into the prepared mug or paper cup, stir together 3 tablespoons of cake mix and 2 tablespoons of water. Drop two chocolate squares, then 1 tablespoon of pudding, into the center of the batter.
Microwave on high for 40 seconds- 1 minute. (The pudding heats quickly and can cause the mug cake to pop. In my microwave 40 seconds cooked the cake, any longer and the cake popped.)
The cake can be eaten right out of the mug/paper cup. If you want to transfer the cake onto a plate, run a knife between the cake and the mug to loosen, then dump out onto a plate. The melted chocolate can be spread with a knife across the top of the cake, to resemble a Boston Cream Pie.
Combine the two cake mixes in a gallon-size ziplock bag labeled with directions. Include a measuring spoon, bag of chocolates, 1 or more vanilla pudding snack packs, and a can of non-stick spray or microwave-safe paper cups.