Soften the butter in a bowl using a spatula. Add icing sugar, vanilla sugar and salt. Mix well. Stir in briefly the sifted flour. Add egg white and knead all, shortly.
Divide dough in 2 parts: a clear natural version and a darkened version by adding chocolate powder. Roll dough into balls (if necessary add a little water or flour depending on consistency). Cover in a food-quality film and chill for 1h.
Lower the dough to max 3mm thick on the lightly floured worktop. Cut them into small shapes with cookie cutters (not to big). Put on baking tray covered by parchment paper. Pop it into the fridge for 20 minutes.
Cook the biscuits in the middle of preheated oven at about 200°C for 5-7 minutes (convection oven if possible). Remove from oven and let it cool on a backing tray. Set aside.
Trick: biscuits can be prepared 1-2 days in advance and stored in a closed box.
For toppings:
Black chocolate: melt chocolate over low heat with cream and liqueur. Incorporate in a chilled bowl, let it cool slightly and add softened butter. Let it cool until the mass is firm but still malleable.
White chocolate: heat cream in a small pan. When boiling, remove from heat and pour on finely grated white chocolate. Mix until it melts and add liqueur. Transfer in a chilled bowl, let it cool a while and incorporate softened butter and the coconut. Chill into the fridge until the mixture consolidates. This mass will tend to stay more malleable than the black chocolate’s one. To solidify a bit the toppings, you can add a little more coconut depending on the consistency.
Spread toppings with a spoon on the biscuits reverse. Cover with the other part of the biscuit and press lightly to make it stick.
For decoration, sprinkle the biscuits with icing sugar. Melt the frosting. Quickly drizzle on side with decorative zigzag and let dry.
