In a large bowl, whisk together the flour, baking soda, cornstarch, and salt until well combined. Set aside.

In a medium bowl, mix the melted butter, brown sugar, and granulated sugar together until smooth. Then, add the egg and egg yolk and whisk until everything is combined. Finish by whisking in the vanilla extract.

Pour the wet mixture into the dry ingredients. Using a spoon or spatula, gently fold the mixture together until just combined. Add the chocolate chips and fold them in until evenly distributed.

Cover the dough and refrigerate for at least 2 hours, or up to 3 days for the best flavor and texture.

Preheat your oven to 325°F (163°C). Line baking sheets with parchment paper to prevent sticking.

Scoop the chilled dough and roll it into balls, placing them on the prepared baking sheets about 3 inches apart.

Bake for 13-14 minutes, or until the edges are lightly browned. Allow them to cool on the baking sheet for about 10 minutes before transferring to a cooling rack.