In a large bowl, whisk together the flour, baking soda, cornstarch, and salt. Set this mixture aside for later.
In a separate medium bowl, whisk the softened butter, brown sugar, and granulated sugar until smooth, ensuring no sugar lumps remain. Add in the egg and egg yolk, then the vanilla extract, mixing until well combined. The mixture will be somewhat thin.
Pour the wet ingredients into the dry ingredients bowl. Stir together with a large spoon or rubber spatula until the dough forms. It will be soft and may seem greasy. Fold in the chocolate chips, trying to distribute them evenly, even if they don't stick well due to the butter.
Cover the dough tightly with plastic wrap and refrigerate for at least 2 to 3 hours, or up to 3 days. For best results and less cookie spreading, chill overnight.
When ready to bake, let the dough sit at room temperature for 10 minutes to soften slightly. Preheat your oven to 325°F and line baking sheets with parchment paper or silicone mats.
Use a cookie scoop or tablespoon to measure out the dough. For extra-large cookies, use 3 scant tablespoons of dough; for medium-large cookies, use 2 heaping tablespoons. Roll the dough into tall, cylindrical balls and place them on the prepared baking sheets, spacing them out to allow for spreading.
Place the cookies in the oven and bake for 12–13 minutes, or until the edges are just lightly browned. The centers will appear soft but will firm up as they cool. For extra-large cookies, you may need up to 14 minutes of baking time.
Allow the cookies to cool on the baking sheet for 10 minutes before transferring them to a wire rack to cool completely. Optionally, press a few extra chocolate chips on top of the warm cookies for a bakery-style look.