Preheat the oven to 175 C.
Line a baking sheet (20 x 20 cm) with baking parchment.
Melt the butter in a pan, remove from the heat and add the chopped chocolate.
Whisk together the eggs, sugar, vanilla sugar and salt in a large bowl.
Add the chocolate and fold in the flour.
Pour the mixture onto the baking sheet and bake for about 10 minutes.
Leave to cool completely.
Place the mixture in a large mixing bowl and add the rum while stirring at low speed.
Keep stirring until the mixture sticks together and you can mould it into balls.
Form into small balls and roll them in the sugar.