In a large bowl whisk together the butter and 100g of sugar until light and fluffy.
Add the egg yolk and the vanilla extract. Reduce the speed of your mixer and gradually add the flour and a pinch of salt.
Wrap the dough into cling film and refrigerate for at least two hours.
Preheat the oven to 160°.
In a small bowl mix the hazelnuts and the remaining sugar.
Using your hands, roll the dough into 3 cm large balls.
Dip them in the egg white, rolling them in order to cover the balls completely. Then, dip the balls into the chopped hazelnuts, coating completely.
Place the dough balls on a baking sheet. Using your thumb, gently press the center of each cookie in to form a central well.
Bake the cookies for 10 minutes.
Take them out of the oven and gently press down the centers of the cookies again. Return them to the oven and continue baking them for another 10 minutes, until golden brown.
Once, taken out of the oven, let them cool for a few seconds on a wire rack.
With a teaspoon now scoop the jam into the center of each cookie.
Enjoy!!!!
