Put flour, sour cream and butter in flakes in a mixing bowl. Knead first with the dough hooks of the hand mixer, then with cool hands to a smooth dough. Roll out the dough on a well floured work surface to a thickness of approx. 0.5 cm.
Cut out circles (approx. 4 cm Ø). Cut out another circle (approx. 1 cm Ø) from the middle of each circle to form rings. (Work the dough very carefully, as it is very fine.) Mix the egg yolk and coat the rings with it.
Sprinkle rings with sugar crystals. Place on 2 baking trays lined with baking paper. Bake the trays one after the other in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 15 minutes.
Remove from the oven, place on a rack and allow to cool.