Begin by sifting the flour, baking soda, cornstarch, and salt together in a large mixing bowl. Set this mixture aside.
In a separate, medium-sized bowl, blend the melted butter, brown sugar, and granulated sugar until the mixture is smooth and free of lumps. Add in the egg and egg yolk, mixing thoroughly, followed by the vanilla extract. The resulting mixture should be thin in consistency. Gradually combine this wet mixture with the dry ingredients using a large spoon or a rubber spatula. The dough will be thick, soft, and somewhat greasy in appearance. Gently fold in the chocolate chips. They might not adhere well due to the melted butter, but ensure they are evenly distributed.
Cover the bowl tightly and refrigerate the dough for at least 2–3 hours, or up to 3 days. For best results and minimal spreading, chill the dough overnight.
When ready to bake, remove the dough from the refrigerator and let it soften at room temperature for about 10 minutes.
Preheat your oven to 325°F (163°C). Prepare large baking sheets by lining them with parchment paper or silicone baking mats.
Use a cookie scoop or a Tablespoon measuring spoon to portion out the dough. For extra-large cookies, use approximately 3 scant Tablespoons (around 2 ounces or 60g) per cookie, and for medium/large cookies, use about 2 heaping Tablespoons (approximately 1.75 ounces or 50g). Roll the dough into balls, ensuring they are taller than they are wide, resembling cylinders. This shape helps the cookies bake up thicker. Place 8–9 dough balls on each prepared baking sheet.
Bake the cookies for 12–13 minutes, or until the edges are just starting to turn lightly golden. (XL cookies might need closer to 14 minutes.) The centers will appear very soft but will firm up as they cool. Allow the cookies to cool on the baking sheet for 10 minutes. During this time, you can press a few additional chocolate chips into the tops of the warm cookies for a more appealing look, although this step is optional. After 10 minutes, transfer the cookies to a wire rack to cool completely.
Store the cookies in an airtight container at room temperature, where they will stay fresh for up to one week.