For the poppy seed croissants, quickly knead all ingredients into a firm dough, making sure the butter is just soft enough to hold the ingredients together. If the dough becomes too soft, shape into a roll and place in the refrigerator, wrapped in plastic wrap, until firm.
Let the dough rest for at least an hour before processing, for this divide it into two portions and form each into a roll with the diameter of about 2 cm. To prevent the dough from drying out, place it in a freezer bag.
Cut finger-thick slices from the rolls, shape into a croissant and place on a baking tray lined with baking paper.
Refrigerate the baking tray with the formed croissants for at least 2 hours. Only then put them into the oven preheated to 160 degrees and bake for about 10 minutes.
Allow to cool slightly, then gently remove from the baking tray and carefully roll the poppy seed crescents in powdered sugar mixed with poppy seeds and vanilla sugar.