Preheat oven to 350 degrees F. Line a baking sheet with parchment paper or a silicone mat.
In a large bowl, cream together the butter, brown sugar and white sugar until light and fluffy. Add the egg and vanilla and beat until smooth. Slowly add in the flour, cornstarch and baking soda, making sure it is well incorporated. Add the chocolate chips and stir to mix throughout the dough.
Scoop out 2-tablespoon portions and roll into balls. Place each ball on the prepared baking sheet. Bake for 11 to 12 minutes. Remove from the oven when the cookies are puffy and barely cooked through - do not let them brown. The cookies should be round and pillow like (and not browned at all) when they're ready to come out of the oven.
Sprinkle lightly with salt, if using, as soon as they're out of the oven. Let the cookies cool for 1 minute on the tray and then transfer to a wire rack. As they cool, they will settle into the dips and ridges that you see in the photos. Store the completely cool cookies in an airtight container for up to one week.