Break up the chocolate, add the butter and melt gently (I do it in the microwave), stirring regularly until melted, mixed and smooth.  Remove and cool for a few minutes

Beat egg yolks and fold these and the cream into the slightly cooled chocolate

Beat the egg whites until soft peaks, then fold into the chocolate mixture.

Spoon or pipe into small ramekins or glasses.  Chill in the fridge until firm