Mix flour, ground hazelnuts, sugar, vanillin sugar and salt in a bowl. Spread butter in small pieces on top, add egg. Knead everything first with the dough hooks of the hand mixer, then with cool hands to a smooth short pastry.
Form dough into a ball, press flat, wrap in foil and chill for about 2 hours.
Roll out the dough in small portions about 3 mm thick on a floured work surface. Cut out stars, hearts or fir trees as desired (stars 6-6.5 cm Ø, hearts: 5 cm Ø, fir trees: 7 cm length).
Knead, roll out and cut out the dough again and again. Place figures on 4-5 baking trays covered with baking paper. Bake the figures in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 10 minutes.
Remove the cookies and let them cool down on a cake rack.
Place the egg whites, icing sugar and lemon juice in a mixing bowl and whisk with the whisk of the hand mixer until a thick icing is formed. Pour the icing in portions into small paper piping bags.
Decorate cooled cookies with the icing. Cut hazelnuts into thin slices and decorate some fir trees with them. Decorate some hearts with colourful sugar pearls. Let the icing dry.
For storage, layer the cookies with baking paper in tins.