Cream together the caster sugar, vanilla sugar and butter. Add the egg and mix until incorporated.
Add the flour and salt and mix to a firm dough. Knead in the chocolate pieces at the last minute. I used dark chocolate and chopped it finely.
Wrap the dough in cling film and refrigerate for at least an hour.
On a floured work surface, roll out the dough to about 5 mm thick. Do not use too much flour, as the recipe already contains a lot of flour, or the cookies will taste too "floury".
Cut out the cookies with a cookie cutter of your choice. Place the cookies on a baking sheet and bake at 180°C/350℉ (conventional oven) for about 10-12 minutes. Small cookies will bake a little faster, so it is best to bake the same size cookies per sheet.
Remove the cookies from the oven when the edges are golden brown. The cookies will still be a little soft when they come out of the oven, so let them cool a bit before placing them on a wire rack.
When the cookies are completely cooled, you can decorate them if you want. With melted chocolate for example.
How to store
Undecorated cookies are best stored in an airtight container and will keep for up to 4-6 weeks. You can learn more about storing decorated cookies here.