Preheat the oven to 350 and prepare a cookie sheet with parchment paper. Set to the side.
In a medium-sized bowl, combine the flour, baking soda, salt, and cornstarch. Set to the side.
In a medium-sized bowl with a hand mixer or in a stand mixer with a paddle attachment, cream the butter with the sugars until light and fluffy, about 2-3 minutes. Scrape the bowl down as needed. Add in the egg and egg yolk, mixing well after each addition. Add in the vanilla. Scrape down the sides as needed.
Gradually add in the flour mixture to the butter mixture, scraping down the sides as needed. Fold in the chocolate chips, making sure they are evenly distributed.
Using a medium-sized cookie scoop, scoop the cookie dough onto the prepared sheet, baking about 6 at a time. Bake the cookies for about 9-11 minutes, until the edges of the cookies are set. Let the cookies cool on the baking sheet for about five minutes before moving to a cooling rack.
