Beat all ingredients together into a thin batter. Cover and let rest at room temperature for 30 minutes.
Preheat the oven to 400 degrees. Place a dollop (a full teaspoon or so) of tallow in each individual muffin tin. You can also bake the Pudding in a large dish–in that case, you need to place several dollops all over the dish. Place empty muffin pan into the oven for 10 minutes, or until the butter is melted and the pan is very hot. Remove the pan and swirl the melted butter a bit so it coats the sides of each muffin cup.
Pour the batter into each muffin cup on top of the melted butter–you want it to sizzle! Do not…I reapeat, DO NOT…put too much batter in each of the muffin cups. This stuff puffs up a LOT…so fill them about 1/4 full.
Bake for 15 minutes, or until the tops are puffy and brown. Tear open to expose the hot, buttery inside and top with roast beef (or turkey) and gravy!