Vareniki (Cherry Dumplings) with Sour Cream

In a large bowl, combine flour and salt. Make a well in the center and add water and egg. Stir with a fork until a shaggy dough forms, then turn out onto a lightly floured surface and knead for about 5 minutes, until smooth and elastic. Cover with plastic wrap or a kitchen towel and let rest for 30 minutes.

While the dough rests, combine the pitted cherries and sugar in a bowl. Let stand for 15 minutes to draw out the juices. Drain the cherry juice into a small saucepan (reserve the cherries), then simmer the juice over medium heat until reduced by half, about 5 minutes. Pour the reduced juice back over the cherries, add the cornstarch, and stir until well combined. Set aside.

Divide the rested dough into two portions. On a floured surface, roll one portion out to about 1/8-inch (3 mm) thickness. Using a round cutter (about 3 inches / 7.5 cm in diameter), cut out circles. Re-roll and cut the scraps. Repeat with the remaining dough.

Place about 1 tablespoon of cherry mixture in the center of each dough circle. Fold the dough over to form a half-moon, pressing out air. Crimp the edges with a fork or pinch with your fingers to seal tightly. Repeat with the remaining dough and filling. If freezing, place on a parchment-lined baking sheet in a single layer and freeze until solid, then transfer to a zip-top bag.

Bring a large pot of salted water to a boil. Carefully drop in the vareniki in batches, making sure not to overcrowd. Stir gently once so they don't stick to the bottom. Cook for about 5 minutes after they float to the surface; they should be tender and slightly puffed. Remove with a slotted spoon.

Serve hot, topped with a generous spoonful of sour cream and a drizzle of melted butter if desired. Optionally, sprinkle with a little extra sugar.