1. Place potatoes in a medium saucepan and add enough salted water to cover. Bring to boiling. Reduce heat and simmer, uncovered and stirring occasionally, until potatoes are tender, about 15 minutes. Drain potatoes and return to saucepan. Add cream cheese and mash until almost smooth. Season to taste with salt and pepper. Let cool. 

2. Meanwhile, in a small skillet melt 4 tbsp. of the butter over medium. Add onion; cook, stirring occasionally until very tender and golden, 8 to 10 minutes. Let cool and stir into potatoes.

3. For dough, in the bowl of a stand mixer fitted with a dough hook, combine flour and 1/2 tsp. salt. (Or use a large bowl and mix with a spoon.) With the motor running on low, add eggs and yolks, 2 tbsp. melted butter, and milk. Add more milk as needed to make a soft, moist dough that is just slightly sticky. Cover dough and chill at least 1 hour and up to 4 hours. 

4. Roll mashed potato mixture into 60 1-inch balls (or scoop using a 2-tsp. scoop). Divide dough into four portions. On a well-floured surface, roll one dough portion to ¹⁄₁₆ inch thick. Using a 2½-inch biscuit cutter, cut out about 15 circles. Repeat with each dough portion.

5. Place a potato ball in center of each dough circle; fold dough circle in half over filling, gently stretching dough as needed. Pinch dough edges together to seal. Transfer to a lightly floured clean kitchen towel.

6. Preheat oven to 200°F. Place remaining 8 Tbsp. butter in a large baking dish; place dish in oven. Bring a large pot of lightly salted water to boiling. Cook pierogi, 10 to 12 at a time, in boiling water until they float, about 5 minutes. Remove dish from oven and use a slotted spoon to transfer pierogi to dish. Tilt dish to coat pierogi with butter; cover and return dish to oven. Repeat with remaining pierogi.

7. If you like (or the day after), in a skillet fry pierogi in additional melted butter until lightly browned.

Smoky Cheddar-Bacon Pierogi

Omit cream cheese. Mash ½ cup sour cream; 1 cup shredded smoked cheddar cheese (4 oz.); 4 slices crisp-cooked bacon, crumbled; 1 Tbsp. chopped fresh chives; and ½ tsp. coarsely ground black pepper with cooked potatoes.

Goat Cheese and Dill Pierogi

Reduce cream cheese to 4 oz. Stir 4 oz. herb-and-garlic goat cheese, 2 Tbsp. chopped fresh parsley, 2 tsp. chopped fresh dill, and ½ tsp. lemon zest into the mashed potato mixture.

Red Pepper and Herb Pierogi

Reduce cream cheese to 3 oz. and mash one 5.3-oz. pkg. semisoft cheese with garlic and fine herbs with potatoes. Stir ½ cup chopped roasted red bell peppers into mashed potato mixture.

To Make Ahead

Pierogi freeze beautifully, so it's easy to assemble them ahead and cook them when you're ready. To freeze the pierogi, prepare through step 5. Arrange on a parchment-lined baking sheet and freeze until firm, about 1 hour. Transfer to a freezer container or freezer bag. Label and freeze up to 3 months. Simmer as directed in step 6, increasing cooking time to 6 minutes or until pierogi float.