Place bacon in a large, deep frying pan. Cook over medium high heat until evenly brown. Drain, reserving bacon fat, crumble and set aside.
In medium bowl, combine sauerkraut, onion powder, crumbled bacon and bacon fat. Stir well; set aside.
In large bowl, beat together 3 eggs, cream, salt, milk and 50g butter with electric mixer. Stir in flour to form a soft dough. Cover and let rest 5 minutes in a warm place. Grease hands and dough with butter and knead on a floured surface 10 minutes. Divide dough into 4 sections and grease each with butter to prevent drying out.
Melt remaining 50g butter; beat remaining egg, combine egg and butter and set aside. Roll out a section of dough to a thickness of 3mm. Cut out 10cm circles with pastry cutter or glass. Place one tablespoon sauerkraut filling in the centre of each circle, brush the edges of the circle with the butter and egg mixture, fold in half and press edges together to seal. Repeat with remaining sections of dough.
Bring a large saucepan of water to the boil. Cook pierogi for 5 minutes or until they float to the top.