Mix the butter with sugar and vanilla sugar until frothy. Add the eggs one at a time and whisk. Mix the flour, salt, cinnamon, oat flakes and baking powder, gradually stir in. Warm the milk and slowly add it with the rum. Roughly grate the apple and fold in.
Bake the dough in portions in the preheated waffle iron until golden.
Sprinkle the still hot waffles generously with powdered sugar and serve.
Makes approx. 18 waffles. The apple only comes into its own very easily. If you are an apple fan, you can add a second apple.