All products need to be at room temperature.
Beat the eggs to fluffy cream with the sugar and vanilla sugar.
Gradually pour in the milk, lemon juice and melted butter, without stopping the beating.
Sift the flour with baking powder in lots to the mixture, carefully stir with a spatula or wooden spoon until all of the flour is absorbed and it forms a homogeneous mixture.
You need to stir in only one direction - clockwise or counterclockwise, and after you complete one revolution, press down slightly in the middle. That's the secret to airy and fluffy muffins. The lemon juice is also key in further contributing to this.
Pour 1 spoon at a time of the mixture into 7-8 muffin forms, floured and greased ahead of time, then pour in a spoon of jam of your choice but without filling them to the top, so that there's room for the muffins to puff up.
Bake in a preheated 356°F (180 °C) oven for about 20 min. or to a dry toothpick.
Once they cool, sprinkle the baked muffins with powdered sugar.