Mix the butter with sugar, vanilla sugar and a pinch of salt until frothy. Gradually add the eggs. Mix the flour with baking powder and cocoa powder and add to the dough spoon by spoon. Finally add the rum and the grated chocolate, stir well and let the dough rest for 30 minutes.
Then bake waffles in the greased waffle iron. Sprinkle with powdered sugar and serve warm.