Prepare the dough. In a bowl, thoroughly mix the water, eggs and salt until smooth, beating them with a fork for about 4-5 minutes.
Start adding flour, stirring the dough with each new portion: first with a spoon, and when it starts to thicken, with your hand. The dough needs to be kneaded thoroughly, taking it from the edges to the center. After that, cover it with a towel and put it in the refrigerator for 30 minutes. During this time, it will become elastic and pliable.
While the dough is in the refrigerator, prepare the filling. Add sugar and salt to the cottage cheese and, if necessary, egg yolk. Mix thoroughly until smooth.
Start making vareniki. Sprinkle the surface of the table with flour, take the rested dough out of the refrigerator and roll it out with a rolling pin to a thickness of about 1 mm. Take a glass or cup and cut small circles. The rest of the dough can be rolled into a "sausage", cut into small pieces and rolled out into circles by hand.
Place the filling in the center of each circle and pinch the edges. The seam should be tight enough to prevent it from falling apart.
Boil the dumplings. Throw them into the water and stir with a slotted spoon so that they don’t stick to the bottom. After about 2 minutes, as soon as the dumplings float to the surface, take them out and serve.