1. Boil the potatoes until they are tender. Drain.
2. Add salt, cheese and butter, then mash together and set aside.
1. Knead the ingredients together. The dough needs to be stretchy but not wet, so add the water a little at a time.
2. Let the dough rest for five minutes.
1. On a floured surface, roll out the dough. Cut dough into 2 inch squares.
2. Place about 1 tsp. of filling in the middle of the square. Fold into triangle shapes ("pillows") and pinch edges together. Place on lightly floured surface.
3. In large pan bring water to a boil. Place the "pillows" in the water. When they rise to the top, the pierogies are done. Drain.
4. Saute butter and onions (if desired) together. Toss with the pierogies and serve.