1Beat the butter, sugar and zest until light and fluffy. Add the yolk until combined, then the flour. If you’re adding ground almonds, adjust the amount of flour respectively.
2Let the ready dough rest in a refrigerator. After about 1 hour take it out and let loosen a little bit in room temperature.
3On a lightly floured surface, roll out a 3 mm thick disk from the dough and cut out cookies, the bottoms as well as the tops. The traditional shape is round.
4Bake in 180 °C for about 10 minutes, or until the edges are just beginning to turn brown.
5Wait until the cookies have cooled completely. Sift the tops with sugar, spoon jam onto the bottoms and assemble.