Mix butter and powdered sugar together into a creamy texture.
Add egg yolks, sour cream and yogurt, or kefir, and mix everything well together.
Finally add the flour, and using your hands knead the dough into a ball.
Cover the kneaded dough with foil and put it in the fridge for a minimum of 30 minutes. You can leave your dough in the fridge overnight or 24 hours if you like.
Once cooled, take half a portion of the dough and roll it flat with a rolling pin. About 1/4 of an inch (0.5 cm) thick. You can use plastic wrap to avoid sticking.
Using the rim of a round glass or circular cookie cutter, cut out circles from your dough.
Place a teaspoon of plum butter onto each circle.
Fold it on three sides into a shape of a triangle.
Place your shaped cookie onto a cookie sheet.
Bake at 350 degrees Fahrenheit or 180 degrees Celsius for 25-30 minutes.