Roughly chop the chocolate. Dice butter. Melt both together in a hot water bath while stirring, pour into a large mixing bowl
Separate eggs. Stir the egg yolks into the chocolate with a whisk one after the other. Sift icing sugar on top. Stir in well with cognac
Beat the egg white in two portions until very stiff. Then fold into the chocolate mixture in portions with a whisk. Chill in a bowl for at least 2 hours
Whip the cream until stiff and decorate the mousse with tuffs, cocktail cherries etc.