In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, brown sugar, and granulated sugar until fluffy and lighter in color. Add the egg and vanilla, mixing on medium-low speed until combined. Scrape down the sides of the bowl as needed.
With the mixer on low speed, mix in the dry ingredients until just combined. Stir in the chocolate chips.
The cookies can be baked immediately, but for best results, chill the dough for at least 30 minutes and up to 48 hours.
When ready to bake, preheat the oven to 350°F.
Using a cookie scoop or spoon, scoop rounded tablespoons of dough, roll into balls, and place them on parchment-lined baking sheets. If desired, press a few extra chocolate chips on top of each cookie.
Bake for 8-10 minutes, until the edges are just barely golden. The centers may look slightly underdone—this ensures soft and chewy cookies. Avoid overbaking.
Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.