Make Dough

1. In a medium mixing bowl, stir together sour cream, scallions, salt & pepper and truffle oil
2. Remove 1 cup of the sour cream mixture to set aside and transfer to a small container. Cover and refrigerate for later use in the recipe
3. In a stand mixer fitted with a paddle attachment, add all-purpose flour. Start mixer on low speed, and add remaining 2 cups of sour cream mixture, salt & pepper, and eggs, added one at a time
4. Mix until it starts to come together, then remove from mixer and incorporate remaining flour into the dough with your hands
5. Wrap in plastic and refrigerate for at least 2 hours

Make Filling

1. In a medium mixing bowl, combine cooled, mashed potatoes, scallions, cheese, truffle oil, salt & pepper. Stir and set aside

Roll, Cut & Fill Pierogi

1. Dust work surface with all-purpose flour 
2. Roll dough into a long rectangle with a rolling pin, about ¹⁄₈" thickness. This can also be done in a pasta maker
3. Take your round biscuit cutter and cut rounds out of the dough. The excess dough can be re-rolled to cut more rounds
4. Place a heaping tablespoon of filling in the center of each dough round
5. Add 2 tbsp water to a small bowl, then crack the egg into the bowl and beat lightly
6. Brush one half of the outer edge of each round with the egg wash. Fold each into half-moons, pressing the edges together to adhere and seal
7. At this point if you don't plan on cooking immediately, liberally coat in all-purpose flour and layer between pieces of waxed paper in a sealable container. Freeze for up to a week

Boil, Brown & Plate

1. Bring a large pot of salted water to a boil. Gently drop in 4-5 Pierogi at a time (if frozen, the Pierogi can go straight from the freezer to boiling water). Boil until they float to the surface, about 60 seconds (frozen may take a few seconds longer)
2. Remove with a slotted spoon onto a clean linen towel-lined plate
3. If cooking all Pierogi at once, they will need to be browned in batches. Start by melting 4 tbsp butter in a large sauté pan over medium-high heat until the milk solids start to brown, about 2-3 minutes
4. Add 6-8 Pierogi to the pan (without crowding). Sprinkle 2 tbsp chopped shallots around the pan. Brown Pierogi about 1 minute per side, flip & repeat to brown the second side
5. Remove to serving plate. Pour excess browned butter and shallots over top. To cook next batch of Pierogi, wipe pan with paper towel and repeat step 3-4
6. Serve hot with the reserved, refrigerated sour cream mixture