Easy Jam Thumbprint Cookies
Cream together butter with sugar, vanilla sugar, salt and egg yolks. Stir together ground hazelnuts and flour. Add the hazelnut mixture to the butter mixture. Shape the dough into two logs about 3 cm (approximately 1 1/4 inches) in diameter. Cover and refrigerate for 2 hours.
Preheat the oven to 200°C (approximately 400 °F). Line a baking sheet with parchment paper. Cut 2 cm (approximately 3/4 inch) slices from the logs of dough. Shape each slice into a ball and press each ball with the handle of a wooden spoon to make a well in the center.
Place the cookies on the baking sheet and bake until golden, about 12 to 15 minutes. Remove and cool on a wire rack.
Gently warm the jam over low heat and stir until smooth. Spoon the jam into the wells in the center of the cookies. Let cool and serve.