Add a pinch of salt to the flour and knead with the butter, poppy seeds, sugar and egg yolk. Then wrap in foil and leave to rest in the refrigerator for 30 minutes.
Preheat the oven to 160 ° C.
Line a baking sheet with parchment paper and form small, as even as possible, croissants and distribute them on the sheet.
Put in the preheated oven and bake for about 20 minutes.
Prepare a mixture of powdered sugar and vanilla sugar in a soup plate and roll the croissants in it while they are still hot.