Note that it's important that all ingredients are cold. Start by mixing the flour, sourdough starter and half of the butter into a dough. Let it rest in the fridge 30 min.

Roll out the dough to a square with an even thickness. Distribute the rest of the butter on one half of the square. Use a cheese grater to cut the butter into thin slices

Fold over the other part of the dough and roll it to an even thickness. Fold the dough into three parts in one direction and one part in the other direction. You should now have six layers.

Wrap the dough in cling film and let it cool in the refrigerator for at least 15 minutes. After that, you can roll the dough into a desired form or thickness.