Mix butter, sugar, lemon zest, salt and yolks until fluffy.
Sift flour, starch and baking powder and then mix with the butter mixture. Add the barberries and knead quickly by hand to form a dough.
Form small balls from the dough and place them on a baking tray covered with baking paper and press them flat with the bottom of a small glass.
Bake in the oven at 160 °C convection oven for 12 -14 minutes.