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 30 minutes.To make the filling. In a large pot of cold water, bring the potatoes to a boil. Salt the water and cook until the potatoes are tender, about 20 to 30 minutes.Drain the potatoes, return the potatoes to the pot and mash over low heat, or mash in the bowl of a stand mixer fitted with the paddle attachment, adding the butter and cheddar cheese. Season to taste with salt and pepper. 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 filling 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, pressing out all the air. Be sure to keep the dough covered as you work work to prevent from drying out. At this point, the Pierogies 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 too cook, bring a large pot of salted water to a boil. Boil the Pierogies in batches for 1-2 minutes, or until they float. Drain.To make the butter sauce. In a large skillet, brown the butter over medium heat, stirring often until the butter is golden and toasted. Add the rosemary and garlic and cook 30 seconds to a minute or until fragrant. Remove from the heat. Season with salt and pepper. Divide the Pierogies among plates and spoon the butter over the Pierogies. Top with cheddar and parsley. EAT!