For the poppy seed crescents, crumble flour with butter and knead well with poppy seeds, sugar and yolks. Wrap the dough in plastic wrap and let it rest in the refrigerator for at least 1/2 hour.
Form rolls of dough about 3 cm in diameter and cut off small pieces. Roll these pieces into croissants and place on a baking tray covered with baking paper.
Bake in the preheated oven at 160 °C for approx. 10 – 12 minutes. Sprinkle the hot croissants with powdered sugar mixed with vanilla sugar.