Melt the butter, combine with the sugar and mix. Add the egg, sour cream and baking soda, and mix again.
Sift in the flour, add the lemon essence and knead the dough.
For the filling, mix the cottage cheese with the condensed milk.
Roll out the dough and cut circles about 8 cm across. Place a slightly rounded tablespoon of the cottage cheese filling in the center of each.
Transfer the pieces to a baking sheet, folding each one in half, and brush the tops with egg yolk.
Bake in an oven preheated to 180°C for 20-25 minutes.
Let the finished sochni cool. Enjoy!