Put 300 g flour, 100 g icing sugar, 1 pinch of salt, flavour, butter in flakes, 1 egg and possibly 1 tbsp. cold water 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 1 hour.
Line two baking trays with baking paper. Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Roll out the dough onto some flour to a thickness of 4-5 mm. Use a cookie cutter (approx. 7 cm Ø) to cut out approx. 30 ice crystals and spread them on the baking trays.
Whisk the egg yolk and milk together. Spread the biscuits with it and sprinkle with decorating sugar. Bake them one after the other in a hot oven for about 10 minutes and let them cool down. Sprinkle with 2 tbsp. icing sugar.