Bring a large pot of salted water to a boil. Add potatoes and cook until tender but still firm, about 15 minutes. Drain and cool.

Meanwhile, prepare dough by combining 1 egg, 1 tablespoon cream cheese, 1 teaspoon milk, and 1/2 cup flour. Once all the flour has been worked in, add another 1/2 cup flour, 1 tablespoon cream cheese, and 1 teaspoon milk. When the dough is well blended, add 1 egg, 1 tablespoon cream cheese, and another 1/2 cup flour; mix well. Finally, add the remaining egg, 1 teaspoon milk, and 1/2 cup flour. Adjust consistency with milk or flour as needed.

On a well-floured surface, roll out 1/4 of the dough to 1/4-inch thickness. Use a rounded cutter to cut out circles of dough. Flour both sides and place on wax paper. Repeat with the remaining dough.

Mash the potatoes with the cottage cheese, salt, and butter. Place a spoonful of filling below the center of a dough circle. Fold the dough over and seal edges carefully.

Bring a large pot of salted water to a boil. Drop in pierogis, a few at a time, and cook for 1 to 2 minutes or until they float. Drain.