"You've probably heard of savory pierogi - those delicious Polish dumplings - but what about sweet? These blueberry-filled pierogi (babcine pierogi z jagodami) are a unique and fruity after-dinner dessert. Serve the pierogi the traditional Polish way: drizzle with melted butter and top with soured cream, then sprinkle with additional sugar and blueberries."
Place flour on a clean work surface and make a well in the center. Crack eggs into the well, add butter, and pour in a little of the warm water. Start mixing together, adding a bit more water if needed. Knead well, continuing to add more water as needed. Continue kneading until dough is soft and smooth, adding a little more flour only if needed.
Dust a work surface with flour. Cut off 1/4 of the dough and roll out to a thickness of 1/8 inch. Cut out circles using a glass or a round pastry cutter.
Fill each dough circle with 1 teaspoon of blueberries and a sprinkle of sugar. Fold dough over into a half-moon shape and seal edges. Set aside on a tea towel and repeat with remaining dough, blueberries, and sugar.
Bring a large pan of salted water to a gentle boil. Add the pierogi in batches and cook until they float to the surface, 5 to 7 minutes. Cook for 2 to 3 minutes once they float to the surface. Remove with a slotted spoon and place in a colander to drain.