Mix all the ingredients for the batter using a whisk or an electric mixer until homogeneous. If you're using a stand mixer, remember to stop every once in a while and scrape down the sides and base of the bowl where dry ingredients tend to accumulate.
If your muffin pan is non-stick, just grease them with a light layer of oil. If they're not, use paper linings. Fill the pan leaving 3mm without covering.
Bake the muffins at 180°C/350°F for 20-30min or until you can insert a knife or skewer and it comes out clean.
Let them cool down completely before unmoulding and especially filling.