Spread about 2 tablespoons of the potato filling on dough leaving the sides empty.
Using a rolling pin, roll each piece into a round-shape crust (size of a medium plate).
Roll the dough out into a log and cut it into 20-24 pieces.
Knead the dough for a few minutes - it will remain slightly sticky but generally elastic and easy to handle if sprinkled with the flour.
To make the dough, in a large bowl mix together the rye flour, milk, sour cream and salt. There is one secret - there are no exact proportions so you may need to add some more flour or liquid (water is fine). I can recommend adding all the liquid ingredients and 3 cups of the flour at once, adding the remaining flour gradually.
Start with the potato filling. In a pot of boiling water cook the potatoes until soft. Drain. While still hot, mash it adding the butter, sour cream, and salt. The potatoes shouldn't remain lumpy. Leave it to cool.