Roughly chop the chocolate and place in a small saucepan with the butter, let it melt over a mild heat or in a water bath.
Separate the eggs. Mix the egg yolks with the sugar until frothy and
add the melted chocolate. Beat the egg whites until stiff and add 2 tablespoons of them to the chocolate mixture and let the whole thing cool.
Whip the cream with the icing sugar until stiff and fold into the cooled chocolate mass together with the remaining egg whites. Pour into a bowl and leave to rest in the refrigerator overnight.
Decorate with a little whipped cream and the chopped pistachios or possibly coffee beans.