Peel the cold potatoes and grate finely. Knead with margarine, sugar, egg, flour, baking powder, salt, vanilla pulp and lemon zest. Wrap the dough in plastic wrap and refrigerate for 30 minutes. Roll out the dough on a floured work surface to about 3 mm thick and cut out 6 cm circles. Pour plum jam into a piping bag fitted with a nozzle and pipe into the center of each circle. Fold up on three sides, shape into a tricorn shape and press together slightly. Whisk egg yolk and milk and brush the cookies with it, sprinkle with hail sugar. Place on 2 baking sheets lined with baking paper and bake in the preheated oven at 180 °C for approx. 12 – 15 minutes. Allow to cool.