Preheat the oven to 325°F (163°C) and line three large baking sheets with parchment paper or silicone baking mats.
In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt.
In a large bowl, beat together the butter, brown sugar, and granulated sugar until creamy. Add the egg, egg yolk, and vanilla extract, mixing until combined.
Gradually mix in the dry ingredients until just combined, then stir in the chocolate chips.
Divide the dough into six portions, shape into mounds, and flatten slightly. Place on baking sheets at least six inches apart.
Bake for 20-25 minutes until the edges are lightly browned and the centers look soft.
Cool on the baking sheet for 15 minutes before transferring to a wire rack to cool completely.