For the Linzer Kipferl, cream together butter, sugar, salt, lemon juice and vanilla sugar. Stir in the egg and fold in the flour.
Use a star-shaped nozzle to pipe the crescents onto the baking sheet and bake at 180 °C for 10 – 15 minutes. After cooling, spread half of the croissants with jam and assemble with the other half.
Dip the tops in chocolate icing.