
The approach is to put a couple of stones in the bottom of a pot to hold a silicone muffin cup off the bottom, add water to the pot, put the batter in the cup, place the cup on the stones, put the top on, put the pot on a stove. When the water boils, the cup is enveloped in steam. Eventually, the batter rises, and when the cake passes the toothpick test, it's done. Turn it out and enjoy.
I had to give this a try. I picked up a pouch of Martha White Blueberry Muffin mix at our local grocery, and assembled all the required stuff in the kitchen.