In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt. In the bowl of a stand mixer, with the paddle attachment, beat the butter, brown sugar, and granulated sugar on medium-high speed until light and creamy, about 2 minutes. Add the egg, egg yolk, and vanilla extract, and continue mixing until fully combined. Scrape down the sides and bottom of the bowl as needed. On low speed, mix in the dry ingredients, then add the chocolate chips and beat just until combined.