For the mousse, break the chocolate coating into pieces and melt slowly in a bain-marie over a moderate heat (it curdles quickly!). Let it cool down. Roughly chop the pastry rolls and mix with the chocolate.
Whip the cream until stiff and allow the vanilla sugar to trickle in. Fold the chocolate into the cream and smooth it down. Chill for at least 3 hours.
Peel mandarins so thickly that the white skin is completely removed. Cut mandarins into thin slices. Caramelise the sugar in a pan until golden. Turn the mandarins in it. Heat the liqueur and pour into a soup ladle.
In front of the guests, carefully light the liqueur in the soup ladle with a long match and pour over the mandarins. Flambé until the flame goes out. If you don't dare to do so, simply pour the liqueur over the mandarins and simmer for about 5 minutes.
Arrange the mandarins on four plates. Using a large spoon or ice-cream scoop, cut out the cams from the mousse and spread them on the mandarins. Drizzle the mousse with liqueur-caramel syrup.