Slowly melt 80 g of dark chocolate in a bowl over a hot water bath.
Fill some melted chocolate into a piping bag.
Decorate the inside of the glass with chocolate.
Put the glass back in the freezer.
Blend the remaining melted chocolate, 2 tbsp of light hazelnut cream, 1.5 dl of milk, 1 tsp of cocoa powder, and ice cubes in a tall blender until smooth.