First, make the sourdough starter. Mix the yeast with a little sugar and flour. Then add half a cup of warm milk and stir. You should get a dense mixture. Let it sit at room temperature to rise.

Dissolve the sugar and the vanilla sugar in the warm glass of milk. Mix the flour with salt, add the sourdough starter and gradually, pour the milk in, stirring continuously.

Add the egg yolks and continue kneading. Then, add the butter. The dough should be softer than the bread dough.
When finished kneading, cover with a towel and let it rest in a warm place until it doubles in volume.

When the dough is ready, divide into three parts, and grease the edges with oil.

Roll out the dough with your hands. Don’t use any flour. Add the filling in the middle and roll it.

Put the roll in a baking tray, greased with oil. Let it rise. Then, brush with egg yolk and sprinkle with poppy seeds. Bake the roll at 180oC, for 45-50 minutes.

When done, remove from the oven, cover with a towel and let it cool on a wooden board. When the roll has cooled, slice it.
You can also dust with powdered sugar.