Place the butter and the chocolate (chopped in pieces) in a heat resistant bowl and melt this slowly au bain marie together. Split the eggs and put the eggwhite in a grease free bowl.
As soon as the chocolate and the butter form a smooth mixture, take of the heat and ad the eggyolks one by one and whisk each one until incorporated. Set aside
Whisk the cream to soft peaks and set aside
Whisk the eggwhites to soft peaks and sieve the iciing sugar over it. Whisk till it is glossy and forms stiff peaks.
Add the whipping cream with the chocolate mixture and fold this until well mixed. Do the same thing with the eggwhites and put the chocolate mousse into bowls to set in the fridge. Leave it there for at least 3 hours