• In a food processor, pulse the hazelnuts until they appear sand like.
• In a stand-mixer, combine butter, nutella, and sugar. Mix until well combined, scraping down bowl to ensure ingredients mix together well.
• Add in hazelnuts and flour. Continue to mix until all incorporated.
• Divide dough into four even sections, roll into logs and wrap well in plastic wrap. Place on a cookie sheet and chill for at least one hour.
• Create balls of cookie dough, using a teaspoon to ensure uniform size. Place dough balls onto a cookie sheet prepared with a silicone baking mat or parchment paper. These cookies do not spread, thus can be placed 1 inch apart.
• Chill prepared dough balls in the fridge for another hour.
• Preheat oven to 320°F and bake cookies for 13-15 minutes, until the bottoms are just beginning to turn golden.
• Allow to cool completely on a cooling rack.

Chocolate Nutella Filling

• In a heat safe bowl over a pan of simmering water, combine the dark chocolate and nutella. Stir frequently to avoid chocolate seizing.
• Remove from heat and allow to cool for 15 minutes. Transfer to a resealable bag and set aside.
• Arrange the cookies on a cooling rack, paired with a cookie of a similar size and shape.
• Fill the center of one of the cookie pairs with a small dollop of chocolate mixture. Allow the chocolate to cool for another 5 to 10 minutes until not quite set. Sandwich the cookies together and allow the chocolate to set completely.
• Cookies can be stored in an airtight container for up to one week.