Toast the hazelnuts lightly, then chop them finely in a blender or food processor.
In a bowl, beat the softened butter, powdered sugar, vanilla sugar and salt with a mixer for about 2 minutes, until light and fluffy.
Add the flour and chopped hazelnuts, then knead until a soft dough forms.
Shape the dough into small balls of about 7–8 grams each and place them on a baking tray lined with parchment paper. Bake in a preheated oven at 375°F (190°C) for 15–17 minutes.
Take the baked cookies out of the oven and let them cool completely. Do not overbake them in the oven so they stay soft and tender on the inside.
Melt the chocolate using any method you like: in the microwave, in a small saucepan, or over a hot water bath. The easiest way is to break the chocolate into equal pieces, place them in a small piping bag, then put the bag into hot water for 5–10 minutes.
Pair up the cookie halves, spreading a little melted chocolate on one cookie and sandwiching it with another.
Place the cookies in the fridge for about 30 minutes so the chocolate can set. Your chocolate hazelnut sandwich cookies are ready to enjoy!