Whisk the flour, baking soda, cornstarch, and salt together in a medium bowl and set aside.

In a large bowl, whisk the melted butter, brown sugar, and granulated sugar together until no sugar lumps remain.

Whisk in the egg, then the egg yolk, followed by the vanilla extract.

Pour the dry ingredients into the wet ingredients and mix together with a large spoon or rubber spatula until just combined.

Fold in the chocolate chips.

Cover the dough tightly and chill in the refrigerator for at least 2 hours or up to 3 days.

Preheat oven to 325°F (163°C) and line baking sheets with parchment paper or silicone baking mats.

Scoop rounded tablespoons of dough, approximately 1.5 tablespoons each, and place onto baking sheets 2 inches apart.

Bake for 9-11 minutes until the edges are very lightly browned but the centers still look soft and slightly underbaked.

Cool on the baking sheet for 10 minutes before transferring to a wire rack to cool completely.