Although chocolate mousse is usually made from dark chocolate it's quite a light dessert as chocolate puddings go because of its airy texture - lighter than petits pots au chocolat, for example.
That means you can pair it relatively easily with a conventional dessert wine though remember if you have a flavour such as orange in the mousse that will tend to knock the same flavour out of the wine. Try:
* a sweet red wine such as Andrew Quady's Elysium Black Muscat, Brachetto d'Acqui or Brown Brothers Cienna Rosso (the surprise winner in the What Food, What Wine competition a couple of years ago
* an orangey Spanish Moscatel such as Moscatel de Valencia or Torres Floralis Moscatel d'Oro which also scored well in the competition.
* An off-dry rosé sparkling wine especially if the mousse is made with white or milk chocolate and served with strawberries
* A raspberry or cherry-flavoured beer (Kriek or Frambozen)
* Ratafia di fragola - a gorgeous Italian strawberry-scented liqueur or other strawberry-flavoured liqueur - served well chilled. The essence of summer.
* A violet-flavoured liqueur, again served chilled. (Think violet creams . . . ).