In a large bowl, combine flour, cocoa powder, salt, cornstarch, and baking soda. Whisk and set aside.

In the bowl of a stand mixer fitted with the whisk attachment, add both sugars and melted butter and whisk together well.

Add the eggs and vanilla extract and mix until just combined.

Switch to a paddle attachment and slowly add in the dry ingredients and mix until incorporated and there are no dry spots remaining.

Add in the hazelnuts and chocolate chips

Fold in until evenly distributed through the dough.

Cover cookie dough in the stand mixer bowl with plastic wrap and place in the refrigerator for at least 30 minutes.

Preheat your oven to 350 degrees F and prepare a cookie sheet with parchment paper or a silicon mat.

Use a large cookie scoop (I used a 4 Tablespoon scoop) and scoop out balls of cookie dough and place about 3 inches apart on the pan. Continue with all the remaining cookie dough.

Bake for 10-12 minutes or until the outsides are lightly browned and set.

Remove from the oven and allow to cool about 10 minutes on the cookie sheet and then remove to a wire rack to cool completely.