Combine the cornmeal, salt and sugar in a heatproof bowl. Stir in the boiling water, 1/4 cup at a time, until the mixture is thick and pasty. Let stand until it "swells" (1-2 minutes). Heat the bacon fat in a large cast iron skillet over medium heat. Stir enough milk into the cornmeal mixture to form a batter which looks like slightly mashed potatoes (it should spread only slightly when dropped from a spoon). Drop the batter by heaping serving spoon fulls (about 2 Tablespoons each) into the hot fat, pulling in the edges of the cakes with a metal spatula to prevent spreading. The cakes should be about 1/4 " thick. Cook the cakes until they are brown and crusty (about 6 minutes per side). Drain the cakes on paper towels. Split each cake and serve hot.