Whisk together flour, cornstarch, baking soda, and salt in a medium bowl. Set aside.
Cream butter, Nutella, and both sugars on medium-high speed until combined, about 2 minutes.
Add the egg and vanilla. Beat on medium-high until light and fluffy, about 4 minutes total, be sure to scrape down the sides of the bowl once.
Reduce to low speed. Add the flour mixture in 2-3 small additions, mixing each until just incorporated. Scrape the bowl several times. Add chocolate chips and mix briefly to distribute.
Scoop dough into 1-inch balls. Chill in the refrigerator on a parchment-lined tray, covered, for 2 hours or up to 3 days.
Preheat the oven to 350°F. Arrange dough balls 2 inches apart on a parchment-lined baking sheet.
Bake 9 to 11 minutes, rotating the pan at 6 minutes, until edges are set and tops are just beginning to brown. Cool 5 minutes on the pan before transferring to a wire rack.
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Notes
Chilling: Refrigerating the portioned dough balls for at least 2 hours produces thicker, chewier cookies and improves flavor. Scoop before chilling, not after, since cold dough is difficult to portion. Doneness cue: Pull the cookies when the edges are set and the tops have just started to brown. They will look slightly underdone in the center, which is correct. They firm up as they cool on the pan. Brown sugar: Do not substitute 100% granulated white sugar. Brown sugar retains moisture and keeps the cookies thick. Light or dark brown sugar both work. Storage: Store in an airtight container at room temperature for up to 10 days, or freeze unbaked dough balls for several months and bake straight from frozen at 350°F for 12 to 14 minutes.