Dissolve the yeast with 1 teaspoon of sugar in lukewarm milk.
Melt the butter in a saucepan and let it cool down.
Put the flour, melted butter, sugar, vanilla sugar and salt in a bowl and stir with the hand mixer. Add the milk-yeast-sugar mixture and knead everything well. Shape the dough into a ball and cover about 120 Let rise at room temperature, during which time the dough should have doubled.
Roll out the yeast dough on a lightly floured work surface or baking mat into a rectangle about 40 + 30 with a rolling pin.
Cut 3cm wide strips from the long side and roll up the ends towards the middle. Place the poppy seed rolls on a baking sheet lined with baking paper. Cover and leave the poppy seed rolls to rest for 30 minutes.
Preheat the oven to 160 ° C or lower heat. Beat the egg yolks with 1 teaspoon of water and brush the poppy seed buns with it. Put the poppy seed buns in the oven and bake for about 20-25 minutes. Let cool on a kitchen rack.
Mix the icing sugar with 1 tablespoon of water until smooth and brush the poppy seed rolls with it.