Preheat oven to 400 degrees F. Boil and mash the potatoes, making sure they are free of lumps. Add the milk, butter, and salt. Add just enough flour to make a soft dough, then lay it on a floured board and roll out quickly and lightly to a half-inch sheet. Cut into rounds; bake about 10 minutes, or until just crisp on the outside. Butter and eat before they fall.