Preheat oven to 350 F.
In a bowl, using a hand mixer beat together butter and sugar for 3-4 minutes, until well combined. Scrape down the sides of the bowl.
In a separate bowl, combine flour, cocoa powder and salt.
Add dry ingredients to the butter/sugar mixture and beat until incorporated.
Gather dough into a ball and wrap with plastic wrap.
Refrigerate for 30 minutes.
Take dough out of the fridge and scoop out balls, about 2 tbsp each. Roll each ball into chopped hazelnuts.
Place on a cookie sheet and and press with your thumb in the middle or with a 1/4 tsp measuring spoon.
Chill cookies for 1 more hour. (or 30 minutes, if you are in a rush)
Bake cookies for 12-14 minutes.
Take out of the oven and if the hole in the middle had risen, press with measuring spoon again. Let them cook on the tray for 10 minutes.
Transfer to a cooling rack to cool completely.
Fill cookies with Nutella.