Dough: In a large bowl, mix together the flour and salt. Add in the sour cream, eggs and melted butter; mix until combined. Transfer the dough to a work surface and knead or about 2-3 minutes, or until a smooth ball forms. Cover tightly with plastic wrap and set aside to rest while you make the potato filling.
Potato filling: Add the potaotes to a small pot and cover with water. Boil over medium-high heat until fork tender, about 10-15 minutes.
Once tender, strain the potatoes and return to the pot. Add the butter, bacon grease, sour cream, and salt. Beat with an electric mixer (or use a potato masher) until smooth. Mix in the cheddar cheese and bacon. Set aside.
Assemble: Cut the dough ball in half. Wrap one half back up with plastic wrap and let rest while you work with the other half of dough.
Lightly flour a work surface and roll the dough out as thin as possible, about ⅛ - 1/16 inch thickness. My circle was about 15 inches in diamater. Use a biscuit cutter or small mouth mason jar lid to cut 2-3 inch circles.
Spoon one teaspoon of potato filling into the center of each circle. Working with one pierogi at a time, moisten the edge of the dough with water using your finger or a pastry brush (I like to set a small glass of water near my work surface for this). Fold the dough in half over the filling and pinch the edges of the dough together. Use you a pastry maker or the tip of a fork to crimp the edges of the dough and seal in the potato filling. As you're working, keep the dough covered with platic wrap or a kitchen towel to keep from drying out. Repeat with the other half of dough.
Boil: Bring a medium-size pot of water to a boil. Add a few pierogies at a time and cook until they float, about 2-3 minutes. If pierogies are frozen, cook 4-5 minutes. Use a slotted spoon to remove to a baking sheet.
Fry: Heat a large non-stick pan over medium heat with ½ tablespoon butter and ½ tablespoon olive oil (add more as needed). Working in batches, add pierogies in a single layer and fry about 1-2 minutes on each side until lightly golden brown.
Serve: Serve pierogies warm with your desired toppings.