All of the ingredients need to be at room temperature.
To get nice and fluffy muffins, you're going to have to forget about your mixer. You have to stir the mixture manually with a wooden spatula or stirrer.
Put the eggs, vanilla and sugar in a bowl. Stir the mixture vigorously until you get a creamy consistency. Pour in the milk, stir and pour in the oil.
Sift the flour with the baking powder and pour them all at once into the mixture. Stir very quickly so that the flour gets absorbed and there's no lumps.
The important thing for these muffins is to prepare the mixture as quickly as possible but don't beat with a mixer! This way your dough will remain airy and fluffy.
You have to get a thick mixture, thicker than the kind used for cakes.
Heat the oven to 392°F (200 °C).
Take a muffin tray, put paper cups in it and distribute the mixture in them, filling them halfway. Don't fill them all the way up because they swell during baking and will spill out of the tray.
From this mixture you're going to get about 15 muffins. My tray had room for 12, so I distributed the rest of the dough into oven-safe bowls and baked in them.
Wash and clean the fruits - I used plums. Cut them into small pieces and put them in each cup on top of the dough. Don't push them, just drop them on top, otherwise the fruits will sink in the mixture.
Put the muffin tray in the preheated oven for 20 min. Check to see if they're ready with a toothpick, although they should be baked by that time and will have obtained a fluffy and soft texture.
Take the muffin tray out and leave the muffins in it for about 15 min. to cool. Sprinkle with powdered sugar at the end if desired.