Preheat the oven to 350°F. Line a baking sheet with parchment paper.

In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt. Set aside.

In the bowl of the stand mixer fitted with the paddle attachment, beat the butter and both sugars on medium speed for 2 minutes until light and fluffy.

Add the whole egg, egg yolk, and vanilla and mix on medium speed for 1 minute until combined and smooth, scraping the sides of the bowl as necessary.

Turn the speed to low and add the flour mixture, mixing until evenly incorporated. With the mixer still on low add in 1 1/2 cups of chocolate chunks until evenly incorporated.

Remove the bowl from the mixer and use a spoon to drop the Nutella by heaping spoonfuls into the cookie dough. Gently swirl the Nutella through the dough using the back end of the spoon or a butter knife. You want to see ribbons of Nutella, so be sure not to overstir.

Using a large (3- tablespoon) cookie scoop, drop the dough 2 inches apart on the baking sheet. Press the additional chocolate chunks on top of the cookies, if desired.

Bake for 9 to 10 minutes, until the edges are set.

Allow the cookies to cool for 3 minutes on the baking sheet, then transfer to a wire rack to cool completely.