Prepare the Dough: Whisk together flour and salt in a large mixing bowl. Make a well, add egg, sour cream, and softened butter. Knead for 5-7 minutes until smooth. Let rest for 30-60 minutes.

Assemble the Filling: In a mixing bowl, combine warm mashed potatoes with cheddar cheese and seasonings. Mix well until cheese is melted and evenly mixed.

Shape the Perogies: Roll out the dough and cut out circles. Place filling in the center, fold over and pinch edges to seal.

Cook the Perogies: Boil salted water, add perogies in batches. Let float and simmer for an additional 2-3 minutes before removing.

Pan-Fry for Extra Crispness (Optional): Sauté boiled perogies in butter until golden brown and crispy on both sides.