This is a lovely, rich chocolate mousse and is very easy to make. The only thing is to have a good timing. You have to work quick and carefully, or the chocolate will set in the egg yolk mixture before you're able to fold in the egg whites.
In another bowl, add half of the sugar to the egg yolks and whisk until pale and creamy.
Melt the chocolate with the butter in a double boiler (au bain-marie).
In another bowl, whisk the egg whites with the rest of the sugar until very stiff. (Make sure you clean and dry your whisk before you start with the egg whites).
Fold the chocolate into the egg yolk mixture, then fold the cream into it. Then carefully fold the egg white mixture into it, in three times. You should never stir or the mousse will lose all of it's airiness!
Turn the mousse into little glasses and let it cool in the refrigerator for three hours.
When ready to serve, top it up with some whipped cream, berries and chocolate shavings.