Put flour, butter and sour cream into a mixing bowl and knead with the dough hooks of the hand mixer to a smooth dough. Roll out to a thickness of approx. 0.5 cm on a well floured work surface.
Use a ring cutter (outer circle 5 cm Ø, inner circle 1.8 cm Ø) to cut out rings, press one side of the rings lightly into decorating sugar. Carefully (as the dough is very tender) place them on three 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 15-18 minutes. Remove from the oven, place on a rack and allow to cool.