Cut the butter or margarine into cubes and place in a bowl. Add vanilla sugar, crystal sugar, salt, lemon zest, 2 egg yolks, 1 egg, flour, rum. Process a smooth dough and put in the refrigerator for one hour.
Shape the dough into smaller balls (the size of a cherry), dip them in the egg white and coat them in ground walnuts.
Place them on a baking sheet lined with paper and bake in a preheated oven at 355 °F / 180 °C for about 20 minutes.
Beat the butter into a foam and gradually, in a thin stream, pour in the slightly warm, melted chocolate. Stick the balls with the cream and you can serve.
