Using a stand mixer fitted with the paddle attachment, cream together the softened butter and powdered sugar until light and fluffy. Then add in the vanilla extract and mix until fully combined.
Fold in the flour, hazelnut flour, and salt until just combined. Then fold in the chopped hazelnuts.
Transfer the dough into an airtight container and refrigerate overnight (but a minimum of 2 hours if you just can't wait).
Line a baking sheet with parchment paper.
On a lightly floured surface, roll out the dough ¼ inch thick and cut into your desired shaped. Place cutout cookies 1-2 inches apart on your baking sheet (they won’t spread much).
Place the baking sheet in the freezer for a minimum of 20 minutes (or the fridge for 40 minutes) before baking to allow the cookies to firm back up. This ensures the cookies won't spread during baking.
While the cookies are chilling, preheat your oven to 350°F.
Bake for 9-11 minutes or until the edges are set and just starting to brown. Bake on the middle rack of the oven and rotate the pan halfway through baking for the best results.
Let the cookies cool on the baking sheet for 2-3 minutes, then remove and let them finish cooling on a cooling rack.
Let the cookies cool completely before decorating.
Dip the cookies ⅓ of the way (or more, you do you) into the melted chocolate or drizzle with chocolate and place back on the cooling rack. Sprinkle with chopped hazelnuts before the chocolate cools and hardens.
Let the chocolate harden completely before removing from the cooling rack.
Store cookies in an airtight container at room temperature for up to a week.