You've probably heard of savory pierogies but blueberry pierogies are quite traditional in the Polish culture. These are at every single one of my fiancé's family parties! Impressing him was #1 so I tried my hand at making Polish dough instead of my go-to pasta dough!
Meanwhile, place the flour on a clean work surface. Create a well in the center and add the egg, oil and salt to the well. Using a fork, beat the wet ingredients together. Pour about 1/2 cup of the warm water into the egg mixture. Using the fork, slowly begin incorporating some of the flour into the wet ingredients. Add 1/4 cup more of the warm water. As the dough begins to come together, use a bench scraper to start incorporating more flour. A shaggy dough will begin to form. Add 1/4 cup more of the warm water and continue to mix the dough. At this point, depending on the temperature and humidity, you may need to add more water; you are looking for a smooth dough that's not too sticky. If you added too much water, sprinkle some more flour over the dough and continue kneading until smooth. Once the dough is formed, knead it until smooth, 3 to 4 minutes. Cover the dough with a damp kitchen towel and let rest 5 minutes.
Fill a small bowl with water. Roll the dough out until it is thin enough to see your fingers through it, about 1/16 inch thick. Use a 3-inch biscuit cutter to cut out rounds. Place 3 to 5 blueberries on one half of each round. Dab a finger in the water and wet the edges of the dough, then fold the dough over and press the edges to seal each into a half-moon. Repeat with all the rounds.
Working in batches, drop the pierogis into the boiling water. Cook until they float, 1 to 2 minutes. Transfer the pierogis with a slotted spoon to a baking sheet or platter and sprinkle with the sugar while they are still warm.
Meanwhile, for the sweet dipping sauce: Whisk together the sour cream, sugar, vanilla paste and salt in a medium bowl. Serve the warm pierogis with the sauce.