To make the dough. In a medium bowl, combine the flour, salt, butter, yogurt, and egg, and mix until combined. Knead the dough for 2-3 minutes. Cover and let sit 20 minutes.In a bowl, mix the potatoes with the cheddar cheese. Roll the dough out onto a floured surface to 1/8 inch thickness. Using a biscuit cutter, cut out 3-inch circles. Spoon 2 teaspoons of the mashed potatoes into the center of each round. Brush the edges with water and fold half of the dough over the filling to enclose it. Press down the edges to seal. At this point, the pierogi can be flash-frozen on a baking sheet for 30 minutes, then transferred to a freezer bag and frozen for up to 3 months. When ready to cook, bring a large pot of salted water to a boil. Boil the pierogi in batches for 2-3 minutes, or until they float, then drain.To make the butter sauce, cook the bacon in a large skillet over medium heat until crisp. Drain. Wipe the skillet clean, then set back over medium-high heat add the butter and and cook 2-3 minutes, until the butter is browned. Stir in the garlic and rosemary. Remove from the heat.Drop the pierogi into the sauce, gently tossing to combine. Spoon the pierogi and sauce onto plates. Top with cheddar and bacon. EAT!