For the Dough: In a large bowl, whisk together flour and salt. In a separate bowl, lightly beat the egg and mix with warm water or sour cream. Gradually add the wet ingredients to the dry, mixing until a shaggy dough forms. Turn onto a lightly floured surface and knead for 5-7 minutes until smooth and elastic. Cover with plastic wrap and let rest for 30 minutes.

For the Filling: Boil the peeled and quartered potatoes in salted water until very tender, about 15-20 minutes. Drain thoroughly and mash until smooth. Add shredded cheddar cheese, cream cheese (or cheese sauce), black pepper, and salt to taste. Mix until well combined and creamy. Let cool slightly.

Assemble the Pierogi: Divide the dough into two portions. On a lightly floured surface, roll out one portion of dough to about 1/8-inch thickness. Using a round cookie cutter or a glass (about 2.5-3 inches in diameter), cut out circles. Place about 1 teaspoon of the potato and cheese filling in the center of each dough circle. Fold the dough over the filling to form a half-moon shape. Press the edges firmly to seal, crimping with a fork for a decorative finish.

Cook the Pierogi: Bring a large pot of salted water to a rolling boil. Carefully drop the pierogi into the boiling water, a few at a time, making sure not to overcrowd the pot. Cook for 3-5 minutes, or until they float to the surface and the dough is tender. Remove with a slotted spoon.

Serve: Serve the cooked pierogi immediately. For an extra touch of flavor, sauté them in melted butter until golden brown after boiling. Top with your desired accompaniments such as extra melted butter, sour cream, crispy bacon bits, or caramelized onions.