For the cookie dough, stir together butter, sugar, kirsch, grated lemon zest, salt and egg until frothy. Add the walnuts. Sieve the flour and cornstarch over it and knead a smooth dough. Possibly add a little more flour. Let the dough cool well for 1-2 hours.
Now roll out the dough on a floured surface 4-5 mm thick and cut out cookies. Place on a baking sheet lined with baking paper and bake at 180 ° C for about 12-15 minutes. Let the cookies cool down.
For the filling, bring the butter, sugar, walnuts, jam and kirsch to a boil in a saucepan, stirring constantly. Let cool for 4-5 minutes. Always generously join 2 cookies together with the mass. Leave something of the mass.
Now dust the cookies with powdered sugar. Glue a halved nut with the rest of the filling onto each cookie.