Roll out Pelmeni/Vareniki Dough, 3 or 4 at a time, to 3-inch circles. Place 1-tablespoon filling on each dough round. Brush edges with water. Fold to form half moons, pinch opposite ends to one another to seal. Repeat with remaining balls of dough. Brush pelmeni with beaten egg white.
Bring large pot full of salted water to boil. Without crowding, drop pelmeni into boiling water, stirring to prevent sticking, until the pelmeni come to the surface, about 4 minutes. Remove with slotted spoon.
Melt butter over medium heat and drizzle over pelmeni. Or serve pelmeni with sour cream. Makes 100 pelmeni.