In a medium bowl whisk together flour, cornstarch, baking soda, and salt.
In the bowl of a stand mixer fitted with the paddle attachment, cream together butter and both sugars until fluffy and light in color. Add egg and vanilla and mix to combine.
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 2, and up to 24, hours.
When you are ready to bake the cookies, preheat oven to 350 degrees F.
Using a cookie scoop or tablespoon, drop dough onto parchment-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. 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.