Decadent dark chocolate mousse

In a glass bowl over a saucepan of simmering water, melt dark chocolate with butter.Separate eggs and beat the yolks.Add the chocolate mixture to the yolks and stir to combine.Beat the egg whites with icing sugar until stiff peaks form.Fold whipped cream into the chocolate mixture and then do the same with the egg-white mixture.Spoon into moulds and chill until set.