Break the chocolate into pieces and carefully melt together with the butter over a water bath. Remove the bowl from the heat, gradually stir in the egg yolks and let cool.
Meanwhile, beat the egg white with a pinch of salt until it is almost stiff, then gradually add the powdered sugar and whip until it is completely stiff. Fold the finished egg whites into the chocolate-butter mixture in portions and refrigerate for at least 3 hours until eaten.
Amarena cherries, spiced pears or orange jelly go very well with this.