My paternal aunt gave me this family recipe over 40 years ago and, while it's time consuming, it's worth every minute. DO NOT skimp and use whole eggs instead of just the yolks. It changes the quality of the dough so much that even store bought will taste better.
Whisk one cup of flour and salt together in a large bowl. Make a hollow in the center.
Whisk together the egg yolks, water and oil. Pour into hollow in flour. Stir in flour from sides, adding more flour until it makes a manageable ball of dough that you can turn out onto a floured board (will most likely still be sticky).
Turn out dough onto counter and knead for 5 minutes or so, incorporating flour as needed. When ready, the dough will feel like a baby's bottom.
Divide dough into 2. Wrap one up with plastic wrap and set aside. Roll the other out on a floured counter. You want it quite thin but when the round is cut out, still with a bit of flexibility to it. This is important because if it is too thick, the dough will be thick and chewy after it's cooked.
Cut into 3" rounds (I use a biscuit cutter). Place about a tablespoon or so of potato filling in the center of each round. Take each round and gently fold dough in half, stretching gently to make sure none of the potato filling is along edges. Dip finger in water and lightly round it around half of the circle and then gently pinch and seal the edge. Set on a flour covered cookie sheet.
Pierogi can be cooked immediately (see below) or cookie sheet can be put into freezer just until the pierogi are surface frozen (about 30-60 minutes). Then they can be dropped into a freezer ziploc without fear of them sticking together.
To cook: Bring a pot of water to a boil. Add pierogi and turn down to a gentle boil. Don't let it boil too hard or it will tear the dough apart. Once pierogi's float to surface, cook for 3 minutes and then remove with slotted spoon to a pan of melted butter. OR place pierogi on plate and drizzle with melted butter. I serve these with sour cream as well.