In a food processor, or a stand mixer fitted with the paddle attachment (or a large bowl + hand held mixer), combine the butter, powdered sugar, vanilla, and salt. Beat on medium-low until combined and creamy, about 1 1/2 minutes.

Add the flour and chopped hazelnuts, and mix until just combined, and the dough starts to clump together, another 2 minutes.

Turn the dough out onto a cutting board, and form into a flat disc with your (very clean) hands. Cut the dough in half. Form each half into a 9″ long log. Wrap each log into plastic wrap, and freeze for 30 minutes until firm. Remove the dough after 15 minutes, and roll again to get a nice round shape (no flat sides!)

Preheat your oven to 350*F. Remove the dough from freezer, and unwrap.

Using a sharp chef’s knife, cut the dough into 1/3″ thick slices. Place cookies on a baking sheet lined with parchment paper or a silicone baking mat (VERY IMPORTANT so they don’t stick!). I usually fit about 20 cookies on a baking sheet, since they don’t spread out much. For best, even results, bake the sheets one at a time.

Meanwhile, melt the chocolate. Fill a small pot with water, about 1-inch deep, and bring to a rolling simmer. Place a small heatproof bowl on top of the pot. Put 3/4 of the chocolate in the bowl, stirring until it almost completely melts. Remove from heat. Add remaining chocolate to the bowl, and whisk in until the chocolate is totally melted and smooth.

Dip the cookies in the chocolate, and place back on the parchment lined cookie sheet. Sprinkle with finely chopped hazelnuts. Allow the chocolate to completely set, about 30 minutes.