Chop the dark chocolate into small pieces and slowly melt it with the butter in a water bath.
Separate eggs. Beat egg whites with half of the sugar until stiff, whip the whipped cream until stiff and keep in a cool place. Beat the egg yolks with 2 tablespoons of hot water or coffee and the remaining sugar until creamy. Fold the melted chocolate into the egg yolk cream. Now just fold in the egg white and whipped cream and fill.
Let it steep in the refrigerator for at least 2 hours (ideally 4 - 6 hours).