In a large bowl, whisk together the flour, baking soda, cornstarch, and salt.
In a separate medium bowl, beat together the softened butter, brown sugar, and granulated sugar until smooth, ensuring no sugar lumps remain. Add the egg, egg yolk, and vanilla extract, mixing until well combined. The mixture will be somewhat thin.
Pour the wet ingredients into the dry ingredients. 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, doing your best to distribute them evenly.
Cover the dough tightly with plastic wrap and refrigerate for at least 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 the oven to 325°F and line baking sheets with parchment paper or silicone baking 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.
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.
Optionally, press a few extra chocolate chips on top of the warm cookies for a bakery-style look. Cool the cookies on the baking sheets for 10 minutes, then transfer them to a wire rack to cool completely.