Line a muffin tin with paper liners. For the dough, whisk the butter, sugar, vanilla sugar, eggs and yoghurt together. Add flour, salt and baking powder and stir to form a smooth batter.
Finely chop the mint. Fold the peeled rhubarb, cut into 1 cm pieces, into the dough together with the fresh mint.
Pour into the molds and bake the whole thing in the preheated oven at 190 ° C on the 2nd rack from the bottom for 20-25 minutes. Shortly before the end of the baking time, I put them one rail higher because otherwise they are too light for me.