Peel and core the avocados and cut into large pieces. Puree together with all the other mousse au chocolat ingredients in the food processor or with the hand blender until creamy.
Roast the hazelnuts in a pan until they start to smell. Place in a clean kitchen towel and rub off the peel. Roughly chop the nuts on a cutting board.
For the chocolate sauce, melt the coconut oil in a small saucepan over low heat. Mix with maple syrup and cocoa powder (Caution: do not put the sauce in a cool place, as the coconut oil will make it firm again).
Pour the chocolate mousse into dessert bowls, drizzle with the chocolate sauce, and serve sprinkled with chopped hazelnuts.