Preparation
Step 1
For pierogi dough, combine flour and salt in food processor; pulse to combine. Add egg, 1/2 cup sour cream and butter and pulse just until dough holds together. Wrap dough in plastic wrap and chill at least 2 hours or up to 24 hours.
Step 2
For filling, cook bacon in large sauté pan over medium heat until crisp. Use slotted spoon to transfer bacon to paper towels. Set aside 1/2 cup of bacon for garnish. Pour off and reserve all but 2 tablespoons of drippings from pan. Add leeks to drippings and sauté over medium heat until softened, 6 to 8 minutes. Transfer mixture to large bowl and let cool 5 minutes. Stir in ricotta cheese, egg yolks and pepper. Stir remaining bacon into leek mixture.
Step 3
Remove pierogi dough from refrigerator and cut in half. Roll first half of dough on a floured surface to 1/8-inch thickness. Use a 3-1/2-inch biscuit cutter to cut dough into circles and transfer to clean cutting board lightly dusted with flour or cornmeal. Repeat with remaining dough. There should be 24 rounds.
Step 5
Place 1/4-cup of bacon filling on one side of each dough circle. Fold dough over filling forming a half circle. Press edges with tines of a fork. Repeat with remaining dough circles and filling.
Step 6
Bring large sauté pan of water to boil over high heat. Add several pierogi to water and adjust heat to maintain a gentle simmer. Pierogi are done when they float to the surface, about 3 to 5 minutes. Use slotted spoon to transfer each pierogi to rimmed baking sheet to cool.
Step 7
For each serving, melt 2 tablespoons of reserved bacon drippings in medium skillet over medium-high heat. Add 6 peirogi to skillet and cook until browned on bottom, about 3 minutes. Turn and cook until browned and heated through, about 3 minutes more.
Step 8
Serve garnished with reserved bacon, remaining sour cream and green onions.