In a large bowl, whisk together the flour, baking soda, cornstarch, and salt. Set aside.
In a medium bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth and free of lumps.Whisk in the egg and egg yolk, then add the vanilla extract.
Pour the wet ingredients into the dry ingredients. Mix with a spatula or large spoon until a soft, thick, and slightly shiny dough forms.
Gently fold in the chocolate chips. The dough may be very soft—it’s expected due to the melted butter.
Cover the dough tightly and refrigerate for at least 2 hours, or up to 3 days. Overnight chilling is best for optimum flavor and texture.
Once the dough is chilled, preheat your oven to 325°F (163°C).Line baking sheets with parchment paper or silicone baking mats.
Scoop about 2–3 tablespoons of dough per cookie.Roll into balls and shape them taller rather than wide (like little dough towers) to help achieve chewy centers.
Place cookie dough balls 3 inches apart on prepared baking sheets to allow room for spreading.
Bake for 13–14 minutes, or until edges are just lightly browned. The centers will look soft and underbaked.
Let cookies cool on the baking sheet for 10 minutes, then transfer them to a cooling rack to cool completely.
While cookies are still warm, press a few extra chocolate chips into the tops for a bakery-style finish.