Preheat the oven to 180 degrees C. (350 Fahrenheit)
Add the soft butter and icing sugar to a bowl and beat them with an electric hand mixer until you get a smooth paste.
Add the egg yolk, and beat for a further 30 seconds until the egg yolk is fully incorporated.
Add one tablespoon of vanilla sugar, vanilla extract and flour and, using your hands, mix all ingredients together until you get an easy-to-work-with-dough.
Cover the bowl with clingfilm, and chill the dough in the fridge for 15-20 minutes.
Grease and flour a rectangle baking tray.
Use about a tablespoon of dough to shape balls , then gently press with your thumb in the middle of the ball. Repeat until all the dough is finished.
Fill with jam, then bake in the oven for 12-15 minutes until the edges are lightly browned.
Remove from the oven and allow to cool completely.
Sprinkle some vanilla sugar over the cookies.