Dumplings stuffed with potatoes and white cheese, i.e. "russian dumplings" (pierogi ruskie).
We call those vareniki. True Russian dumplings, are more circular in shape, and made only with ground meat. We call those - pelmeni.
Vareniki (crescent shaped pelmeni with potatoes instead of meat), are traditionally considered a Ukrainian recipe. They're also sometimes made with cherries, or something else sweet.
In the picture below, pelmeni are on the right, and vareniki on the left.
My theory, based on nothing, is that these come into Russian and Ukrainian cuisine from Asia. Maybe from the Tatars, or some of the other Steppe peoples. Looks too much like their dumplings to be a coincidence.