PREPARE DOUGH: Combine dough ingredients, knead till it does not stick to the hands. Divide into four lots, roll them out thinly one at a time. Cover bowl while working to stop dough drying out. Cut rounds, about 2 to 2.5 inches.
MAKE THE DUMPLINGS: Mash all ingredients together to blend well. Place a scant spoon of filling on each round of pastry, fold over to get a half-moon shape. DO NOT OVERSTUFF: the filling expands! Press edges down securely -- I use a fork to seal them so they don't come unstuck in the pot. Every cook has a signature way of sealing!
COOK: In a shallow wide pan or wok, bring salted water to the boil. Drop batches of pierogi in. When they rise, cook for about 4 minutes. Get them out with a strainer spoon, drain well. Place in warmed dish.
SERVE: Pour melted butter or pork dripping with crackling bits over pierogi. Serve piping hot. Optional: serve sour cream on the side.
REHEATING: Any pierogi left over (unlikely) can be warmed through in a frying pan with a little fat if needed.
TO FREEZE: Cook first! The dough is too soft to handle raw.