In a medium bowl whisk together flour, cornstarch, baking soda, and salt.
In the bowl of a stand mixer fitted with the paddle attachment, beat together butter, brown sugar and granulated sugar until fluffy and lighter in color. Add the egg and vanilla and mix on medium-low speed until combined, scraping down the sides of the bowl as needed.
With the mixer on low speed, mix in the dry ingredients until just combined. Stir in chocolate chips.
Cookies may be baked right away, or if you have time, chill the dough for at least 30 minutes and up to 48 hours.
When you are ready to bake the cookies, preheat oven to 350° F.
Using a cookie scoop or spoon, scoop rounded tablespoons of dough, roll into balls and place on parchment paper-lined baking sheets. Press a few extra chocolate chips into the top of each cookie, if desired.
Bake for 8-10 minutes, until barely golden brown around the edges. The centers won't look quite set. Be careful not to over-bake. For the softest cookies you want to slightly under-bake.
Let cookies cool on baking sheet for 5 minutes, then remove to a wire rack to cool completely.