Work the butter, sugar, vanilla sugar, salt and espresso powder with the dough hook until smooth. Briefly mix in the egg yolks. Add flour and hazelnuts and knead into a smooth dough. Wrap in foil and refrigerate for at least 2 hours.
Shape the dough into cherry-sized balls and place on the baking sheets. Make a well in the balls with the end of a wooden spoon and add the jam. Bake in a preheated oven at 180-200 degrees for 10 minutes. Sprinkle with powdered sugar as desired.