Pierogi Dough: Place the flour on a clean work surface. Make a well in the centre. Crack in the eggs, add the butter and pour in a little of the warm water. Start mixing, then add a bit more of the water. Knead well, continuing to add more water as needed. Continue kneading until the dough is soft and smooth, adding a little more flour only if needed.
Working with 1/4 of the dough, roll out to a thickness of 3mm on a floured work surface. Cut into rounds using a glass or round pastry cutter.
Filling: To each round add a teaspoon of blueberries, then sprinkle with sugar. Fold dough over into a half-moon shape and pinch edges together to seal. Set aside on a tea towel and repeat with remaining dough and berries.
To Cook: Bring a large saucepan of salted water to the boil. Once boiling, gradually add the pierogi. Once they float to the surface, cook 2 minutes longer, then remove with a slotted spoon and place in a colander to drain.
To Serve: Serve the pierogi warm with a drizzle of melted butter and topped with sour cream, then sprinkle with additional sugar and blueberries.