Peel cold potatoes, grate them finely or press them through the potato press. Knead with butter, sugar, egg, flour, baking powder, salt, vanilla pulp and lemon zest to form a smooth dough. Chill the dough for 30 minutes.
Preheat the oven to 175 ° C. Line the baking sheet with parchment paper.
Roll out the dough to a thickness of approx. 3 mm. Cut out circles (6 cm diameter - wine glass!) And place on the baking sheet. Spread some plum jam on each circle, leaving about 1 cm free at the edge. Fold up circles on three sides. Firmly press the sides of the dough together into a three-cornered hat.
Mix egg yolks with milk. Brush the cookies with it and sprinkle with sugar. Bake in the oven for about 12 minutes until golden.