110g plain dark chocolate (at least 50% cocoa mass)
Separate egg yolks from whites and keep in separate bowls.
Melt chocolate and butter over low heat. When well mixed remove from heat, add beaten egg yolks one at the time stirring all the time.
Beat the egg whites in a separate bowl until stiff, fold in the sugar and continue to whisk for another few seconds. Add about a third of this mix to the chocolate and mix it well. Then gently pour half the chocolate mixture over the egg white mix and carefully fold in, then repeat with the remainder of the chocolate mix. Put in fridge to cool. (Or put it in the freezer to make it ice cream.)