Cream soft butter with powdered sugar, vanilla sugar, lemon zest and salt until very fluffy. Beat in egg and yolk one at a time, slowly.
Sift together the plain and plain flour and mix in gently.
Fill the dough mixture into a piping bag with a star-shaped nozzle and pipe nicely shaped cookies onto a baking tray lined with baking paper.
Bake in the oven preheated to 200 °C for about 12 minutes (depends on the size of the pieces, keep watching).
When the piped cookies have cooled, spread jam on two pieces at a time and place them together.
Dip the piped pastry in chocolate as desired and let dry on a wire rack.