1.Prepare cake as directed on package for a 9 x 13 pan.
2.Cool in the pan for 14 to 17 minutes. Using the handle of a wooden spoon, poke holes all over the cake going all the way down to the bottom.
3.Combine the pudding mix and cold milk in a large bowl. Whisk for about two minutes. Pour half of the pudding over the cake. It should run down to the bottom.
4.Place the remaining pudding in fridge for about 5 to 7 minutes to thicken up just a bit. Take out and stir.
5.Mix your desired amount of marshmallows into the pudding. Pour over cake then spread over cake as a frosting.
6.Place in fridge for 1 to 2 hours. Enjoy!