Roughly chop the chocolate. Melt 150 g chocolate in a bowl over a warm water bath. Cream fat, sugar and vanilla sugar with the whisk of the hand mixer. Stir in the eggs one after the other.
Stir in crème frâiche, liquid chocolate and whiskey. Mix flour, cocoa, chopped chocolate, espresso powder and baking powder and stir into the dough. Grease a box form (2 litres capacity; 30 cm length) and dust with flour.
Pour in the dough, smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 60-65 minutes. Let the cake cool in the tin for about 10 minutes. Then turn out onto a cake rack and let it cool down for about 2 hours.
Mix icing sugar, lemon juice and food colouring to a thick icing. Fill the icing into the piping bag, cut off a small tip and decorate the cake with lines, let the icing set.
Cut cake into pieces, arrange on a plate and decorate with mint.