1. In a large mixing bowl, combine the flour, baking soda, salt and cornstarch. Set aside.
2. In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld electric mixer, beat the butter for about 1 minute, or until fluffy and whipped. Add both sugars, mixing until combined. Add the egg and vanilla extract and mix until combined. With the mixer running on low, slowly add the dry ingredients into the wet, mixing until just combined. Add the chocolate chips and either mix by hand with a large wooden spoon (my preference) or mix just until combined. Cover the dough tightly and chill for at least 1 hour, and up to 3 days.*
3. Preheat the oven to 350 F and line two large baking sheets with parchment paper or a silicone baking mat. Using about 1.5 tbsp of dough at a time, roll the dough into balls and place them on the cookie sheet about 2 inches apart. I had 12 on each tray. Bake the cookies for 10-11 minutes, removing them from the oven as soon as the outsides are light brown; they will look underdone in the middle, but that’s what you want!
4. Burn your tongue and possibly your fingers by eating a few cookies RIGHT out of the oven. 
5. Leave the remaining cookies on the tray for 5 minutes before removing to a wire rack to cool completely.

Cookies stay fresh at room temperature, in an airtight container, for up to 2 weeks. Cookies can also be frozen, for up to 2 months; see Notes for instructions on freezing the dough.