Put the flour, sour cream and butter in flakes in a mixing bowl. Knead into a smooth dough with cool hands.
Roll out the dough to a thickness of approx. 5 mm on a well-floured work surface. Then cut out round cookies (approx. 4 cm in diameter) with a cookie cutter. Cut out a circle (approx. 1 cm in diameter) from the middle to make rings.
Whisk the egg yolk and coat the rings with it. Sprinkle the rings with crystal sugar. Place on 2 baking sheets lined with baking paper. Bake tray by tray in a preheated oven at 175 degrees for about 15 minutes. Take out of the oven, place on a rack and let cool down.