Put flour, sugar, vanilla sugar, 1 pinch of salt, 1 egg and butter in flakes in a mixing bowl. Knead first with the dough hooks of the mixer, then with your hands to a smooth dough. Wrap in foil and chill for about 30 minutes.
Drain the cherries and chop finely. Mix the poppy seed filling and 1 egg. Halve the dough and roll out between two sheets of baking paper to form a rectangle (approx. 18 x 30 cm). Remove the upper layer of baking paper, spread each sheet of dough with half of the poppy seed mixture, leaving a margin of about 1.5 cm on the lower long side.
Sprinkle half the cherries on each. Roll up from the long side using the baking paper. Roll both rolls in sugar crystals. Chill for about 1 hour.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3). Line two baking trays with baking paper. Cut each roll into approx. 30 slices and distribute on the trays. Whisk the egg yolk and 1 tbsp. water and spread on the cookies.
Bake in a hot oven for 12-14 minutes one after the other. Let them cool down.