Wash the blueberries and drain well. Preheat the oven to 180 °C (convection oven 160 °C) and place the paper molds on the 12 wells of the muffin tray.
Using an electric hand mixer, beat together the butter, ricotta, sour cream, grated lemon zest, vanilla sugar and salt. Stir in the egg, milk, and honey. Add flour and baking powder and mix, lastly fold in the blueberries.
Fill the paper cups 2/3 full with the blueberry mixture. Place the baking tray on the middle shelf in the preheated oven and bake the muffins for 20 minutes.
Remove the finished muffins from the baking sheet wells, but not from the paper cups.
Brush each muffin with a little oil. Let cool well and dust with powdered sugar to taste right before serving.
Blueberry muffins are the classic American fruit mini-cake.