For the avocado-lime mousse, first soak the gelatin in cold water according to package directions.
Peel the avocado, remove from the stone, coarsely dice and finely mash with 2 tablespoons lime juice and 1 teaspoon lime zest.
Squeeze the gelatine well, put it in a bowl with the cognac and dissolve it in a hot water bath. Mix the dissolved gelatine first with just a little avocado puree, then fold into the entire puree. Fold in 1 tablespoon of crème fraîche.
Pour the avocado cream into a shallow dish or immediately portioned into glasses and allow to set in the refrigerator for about 3 hours.
To serve, sprinkle cocoa nibs over the avocado-lime mousse.