Cream the softened butter with the powdered sugar, vanilla sugar, and salt until the mixture is light and fluffy.
Add the egg and mix well until fully incorporated.
Gradually add the flour and mix until you get a smooth, pipeable dough.
Fill the dough into a piping bag with a star nozzle. Pipe small circles, about 5 cm in diameter, or rose shapes onto a baking sheet lined with parchment paper. This recipe makes approximately 20 cookies.
Press a small indentation in the center of each cookie and add jam.
Immediately place the cookies into the preheated oven and bake at 180°C (350°F) top/bottom heat or 160°C (320°F) with fan for about 12 minutes. Ensure the cookies stay light in color, only turning lightly golden on the edges.
Remove from the oven. The cookies will be soft right after baking. Allow them to cool completely as they will firm up while cooling.
