In a large bowl whisk together the flour, cocoa, cornstarch, baking soda and salt. Set aside.
In a separate large bowl, beat together the butter and sugar until fluffy. Beat in the Nutella, followed by the egg and vanilla.
Turn the mixer down to low and add in the flour mixture about 1/2 at a time.
Turn off the mixer and scrape down the sides of the bowl using a large wooden spoon or rubber spatula. Stir in the chocolate chips.
Cover the bowl with plastic and refrigerate for 4 hours, or overnight.
When ready to bake, preheat the oven to 350F degrees. Line 2 cookie sheets with parchment paper or silicone baking mats.
Form the cookies into balls about 1 - 1.5 tablespoons in size (I recommend using a cookie scoop), and place the cookies 2 inches apart on the lined cookie sheets. If the dough is too hard to form into balls, let it sit on the counter for 10 minutes first.
Bake 1 tray at a time in the preheated oven for 11-13 minutes, or until the tops look set.
Remove from the oven and allow the cookies to cool fully on the cookie tray. Optionally, dot the tops of each cookie with extra chocolate chips.