Whisk flour, cocoa, baking soda, and salt in a small bowl. Set aside.
Beat butter and sugars in the bowl of a stand mixer fitted with the paddle attachment until light and fluffy (approx 3mins).
Add Nutella and beat until combined.
Add egg and vanilla and beat until well combined (approx 1 min).
Reduce mixer to low and add flour mixture. Mix until just incorporated.
Add chocolate chips and chopped hazelnuts and mix until just incorporated.
Using a large cookie scoop, portion the dough into 16 mounds. Round the mounds into balls and press down slightly to flatten.*
Place mounds on a tray, cover with plastic wrap, and refrigerate at least 1 hour. Do not bake unchilled dough! Cookies will spread too much.
Preheat oven to 350F. Line a baking sheet with parchment or a Silpat.
Place cookies on baking sheet at least 3" apart. I use a half sheet pan and baked 6 cookies per sheet.
Bake for 11-14minutes or until cookies start to crack.**
Cool on baking sheet for 10mins and transfer to a cooling rack to cool completely.
Notes
*The dough will be quite soft at this point, I placed the scooped balls in the fridge for 15mins to firm up before rounding. **If you prefer fluffier cookies, bake for around 11mins, before cracking.