Dilute the yeast in 100 grams of warm milk, add 1 tablespoon of sugar, sift 2 tablespoons of flour, mix, cover and leave for 20 minutes
Beat the egg with sugar by hand. Sift the flour into a deep bowl and mix with the melted butter, egg-sugar mixture, salt, remaining milk and sourdough.
Knead the dough, cover with a towel and leave for 1 hour. The dough should increase in volume.
As soon as the dough rises, press it down again.
Divide the dough into 2 parts and roll out into 2 layers 2 mm thick.
Cut each layer into triangles.
From the wide side of the triangle, lay out 1.5 tbsp of filling.
Roll each triangle into a roll. Cover the baking sheet with baking paper, lay out the blanks. Leave on for 10-15 minutes.
Preheat the oven to 175 degrees and bake for 20 minutes. Sprinkle the cooled croissants with powdered sugar.
