In a large bowl, combine flour and salt. Gradually add cold water and knead until a smooth dough forms. Cover and set aside.

In a separate bowl, mix mashed potatoes with grated cheese, pepper, and melted butter.

Divide the dough into small portions and roll each into a 3-inch circle.

Place a spoonful of potato and cheese mixture onto each dough circle. Fold and seal the edges firmly.

Bring a large pot of salted water to a boil. Add pierogies and cook for about 3-4 minutes until they float to the top.

Remove with a slotted spoon and fry in a skillet with a bit of butter until golden brown.