Potato and Aged Cheddar Perogies

Name: Potato and Aged Cheddar Perogies
Ingredients: For the dough; 2 cups all-purpose flour; 3/4 teaspoon salt; 1/4 cup melted butter, cooled; 1 egg; 1/2 cup sour cream; For the filling and assembly; 1 pound Yukon Gold potatoes; 3 tablespoons butter; 5 1/2 tablespoons sour cream; 1 1/2 teaspoons salt; 2/3 teaspoon freshly ground black pepper; 2/3 cup aged cheddar, grated; glass of water; butter, for frying