Cream the butter with the powdered sugar. Add vanilla sugar and grated lemon zest. Mix in the egg with a tablespoon of warm water. Stir in the flour. Using a piping bag with a star-shaped nozzle, pipe the mixture onto a baking tray lined with baking paper. Bake in the preheated oven at 200 °C for approx. 10 minutes until golden brown. Allow to cool, spread half of the croissants with apricot jam and assemble the croissants. Dip the ends of each croissant in warmed chocolate.