For the buckwheat cookies, cream butter and sugar until fluffy. Stir in the yolks one at a time. Add flour, salt, baking powder and vanilla sugar and mix to a dough.
Line a baking tray with baking paper. Using a teaspoon, cut off pieces of dough and roll into balls in your hands. Place on the baking sheet, spacing them apart, and flatten slightly. Bake in the preheated oven at 170 °C for 15-20 minutes.