Sprinkle about a pinch of flour on to 2-3 large plates. Take 2-3 other large plates and smear about a teaspoon of butter on each. Set plates aside.
On your dough rolling surface ( a large wooden or marble board), pour flour on board and make a small dent in the middle (a hole with some flour still on the bottom).
Take a cup of warm water and melt 2 tbsp butter in it. Set aside.
Whisk an egg and then with a fork, whisk it in little by little into the flour (see reel above for a demonstration). As you whisk in the egg to the dough, make sure the egg is coming into contact with the flour (and not the board, otherwise it may stick).
Next slowly pour the butter water mixture into the flour, taking care to mix it all with a fork. Once done, start kneading the dough with your hands so that it comes together in a relatively smooth ball.
You might need to add 2 tablespoons additional flour to the dough so it doesn't get too sticky. But as you kneed, don't add too much flour otherwise it will harden the dough but if that happens, just add some water and knead.
When dough starts getting smoother (see photo), cut in half and put a little flour on bottom and wrap in plastic wrap while working with the first half.
Sprinkle some flour on the half ball of dough and take a rolling pin and roll the dough (this will take several minutes) until it gets to a thickness of 2-3 mm.
Once the dough is thin enough, take the open end of a glass or biscuit cutter and cut out circles for the pierogi. Tip: make sure you're cutting out circles that are very close to each other (but no too close) so that you maximize space. With the leftover dough, roll it up and set aside (you'll be rolling it out for more pierogi in a bit).
Use a small spoon to add the potato cheese filling to each circle. Add it to the middle of the pierogi and then gently with your hands, take the opposite ends of the circle and pinch them together with your fingers so that you have a nice semicircle shape. Add it to a large plate with some flour sprinkled on it (so that the pierogi don't stick). Continue with the rest of the circles.
In the meantime, boil a large pot of water. Once it's boiled and you have a set of pierogi to boil (around 12), gently place the pierogi into the water. You don't want too much overcrowding so adding 12 pierogi at a time is just right. Leave them boiling for a few minutes until they all float to the top and look puffier (3-4 minutes). Once done, take a strainer and drain them and place them on a buttered plate (the butter will help separate the pierogi so they don't stick).
Repeat the process: roll out the remaining dough, cut circles, fill and pinch them into the pierogi shape, and boil them in the pot.
You should have around 70-75 pierogi in the end. If you want to save them for later, place them in between layers of wax or parchment paper in an airtight container and freeze or refrigerate (if using the next day or two). If you're ready to eat them, you can cook them until crispy and golden brown or you can do this along with some caramzelid onion and bacon. For the latter, see directions below. In either case, the pierogi will need to be lightly fried so that they get that golden brown color and crispy texture. Take a sauté pan with 1 tbsp oil and heat to medium high. Place however many pierogi you'll be serving in the pan and cook until golden brown (4-5 minutes). Flip and repeat.
If you'd like to serve the pierogi with onions and bacon, dice an onion and cook in a medium heat pan with 1 tbsp of oil until onions are golden brown. Add to this diced bacon strips and sauté until cooked through.