Make the dough by mixing together the flour and salt.
Add the beaten egg to the flour and combine. The dough will be quite clumpy at this stage.
Gradually, work in the sour cream and softened butter until the dough comes together in a slightly rough and sticky ball.
Using just your fingertips, knead and fold the dough without adding additional flour until the dough becomes less sticky but still very moist.
Wrap the dough in plastic wrap and refrigerate for 30 to 60 minutes, or up to 48 hours.
Make the filling by combining the mashed potatoes and shredded sharp cheese.
Stir and mash until the cheese and filling are well combined. Taste and season with sea salt and freshly cracked pepper, to taste.
Fill the pierogi by rolling half the dough 1/8" thick. Use a 3" round cutter to cut circles of dough. Repeat with the other half of the dough.
Place 2 teaspoons of filling on each round of dough.
Wet the top of each pierogi wrap with a little water to help them seal.
Gently fold the dough over, forming a pocket around the filling.
Pinch the edges of the pierogi well to seal, then seal again with the tines of a fork.
Side Note: At this point, the pierogi can be frozen for up to 4 weeks, or refrigerated overnight. To cook pierogi, place the pierogi in a very large pot of boiling salted water.
Only cook about 10 pierogi at a time, so that they have room to cook without sticking.
When the pierogi float, they are done. The time will vary depending on if they are fresh or frozen and how big they are. Mine took only a few minutes.
Finish the pierogi by heating the butter in a large skillet over medium-high heat.
Add the shallots and cook for 1 minute.
Add the drained pierogi and cook until browned and crisped.
Serve plain or with sour cream. Enjoy!