In a large bowl combine flour and salt.
Add egg, 4 tbsp butter, and sour cream. Combine until dough starts for form.
On a well floured surface lightly knead dough just until it comes together, less sticky but still moist.
Wrap dough tightly in plastic wrap and rest in refrigerator for 30 minutes up to 2 days.
Cut dough into four pieces keeping the remaining three wrapped.
Roll out dough into an 1/8" sheet.
Using a 2" cookie cutter or wide mouth glass and cut rounds.
Fill each round with a spoonful of mashed potato, wet half of the edge of the wrapper and pinch the close (or use a fork).
Pierogies can be cooked immediately or frozen for 4 weeks.
Add small batches of pierogies to boiling water for 2 minutes or until they float.
Immediately transfer to a saucepan with 4 tbsp melted butter to fry.
Once pierogies have a golden brown crust on both sides remove from pan and add slices shallots.
Serve immediately with crispy shallots and horseradish aioli (4:1 mayo and horseradish, splash of lemon juice)