Wash fresh blueberries and drain well. Defrost frozen blueberries and drain well without breaking them. Preheat the oven (electric cooker: 190 °C). Butter a 12-piece muffin tin. Melt 125 g butter. Using a whisk, stir the sugar and vanilla sugar into the hot butter, then stir in the eggs and milk.
Mix all the dry ingredients (flour, baking powder, salt) and add to the liquid ingredients, mix very gently and then carefully fold in the blueberries. Tip for defrosted berries: Dust with flour, mix carefully and only then fold in.
Spread the dough evenly on the muffin tin. Bake the muffins in the oven for approx. 18 minutes and then make a cooking test. (As a cooking test, prick a muffin with a wooden skewer. If the skewer stays clean, the muffins are ready. Stick a little more dough on them, put them back in the oven and bake them a few minutes longer :-).