Mix flour, baking powder, vanilla sugar and sugar in a mixing bowl. Mix 230 g buttermilk and liquid butter. Stir the buttermilk butter and eggs into the flour mixture with the whisk of the mixer. Wash the blueberries, drain and fold into the dough.
Preheat the waffle iron and brush with oil. Spread approx. 1-2 tbsp. of dough per wafer on the baking surface. Bake the waffles until golden brown, remove carefully and let them cool down on a cake rack. For the icing, stir 1 tbsp. buttermilk with icing sugar until smooth. Drizzle over the waffles.