Clean, wash and cut the rhubarb into small pieces. Put the fat, salt, vanilla sugar and sugar into a mixing bowl, whisk with the whisk of the hand mixer until foamy. Stir in the eggs one after the other.
Mix flour (up to 1 tablespoon) and baking powder and stir in alternately with the milk. Turn rhubarb in the rest of the flour and fold into the dough. Place 2 paper cups in each muffin pan and spread the dough evenly with a tablespoon.
Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 25-30 minutes. Remove from the oven and allow to cool on a rack. Mix icing sugar and lemon juice and spread on the muffins.