Grandma Urbanski's Potato Pierogi

To make the dough, combine the flour and salt in a large mixing bowl. Make a well in the center and add the water and egg yolks. With a wooden spoon, stir constantly until the dough holds together, adding a little more water if necessary. Turn the dough onto a lightly floured surface and knead it for 2 minutes. Put the dough back in the bowl, cover it with plastic wrap, and set it aside. For the filling, boil the potatoes in a large saucepan of salted water for about 10 minutes or until the potatoes are soft. Drain the potatoes, then mash them by hand. Meanwhile, melt 2 tablespoons of butter in a small skillet. Add the onion and sauté for 5 minutes, stirring often. Add the onion, cheese, and salt to the potatoes and mash them in. Bring a large pot of salted water to a boil. Meanwhile, break the dough into quarters. Roll one out thin on a floured surface. Using a biscuit cutter or a water glass, cut the dough into 3 to 3 1/2-inch circles. Roll out and cut the scraps. Repeat with the other 3 quarters until all the dough is used. Put a heaping teaspoon of filling in the center of each circle. Moisten the edge of the circle with a wet fingertip, then fold it in half and press the edges together firmly to seal. Drop the pierogi, 8 to 10 at a time, into the boiling water. Simmer until the pierogi rise to the surface, then continue to simmer for 4 to 5 minutes more. Next, melt a little butter in a large skillet. Using a slotted spoon, transfer the pierogi from the water into the skillet. Cook them in the butter over medium high heat for several minutes, until lightly browned on both sides. Serve hot and garnish with a little extra cheese, if you like. Makes 4 to 5 dozen.