Put flour, 100 g icing sugar, salt, flavouring, butter in flakes, egg and possibly 1 tbsp. cold water into a mixing bowl. Knead first with the dough hooks of the hand mixer, then with your hands to a smooth dough. Wrap in foil and chill for about 1 hour
Line 2 baking trays with baking paper. Roll out the dough on some flour 4-5 mm thick. 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 one after the other in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 10 minutes. Let them cool down. Sprinkle with 2 tablespoons of icing sugar
Waiting time approx. 1 hour