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

In a medium bowl, whisk the melted butter, brown sugar, and granulated sugar until smooth and free of lumps. Add the egg and egg yolk, whisking until fully combined, then whisk in the vanilla extract. The mixture will be thin.

Pour the wet ingredients into the dry ingredients and mix using a large spoon or spatula until just combined. The dough will be soft, thick, and glossy. Gently fold in the chocolate chips.

Cover the dough tightly and refrigerate for a minimum of 2 hours and up to 3 days. Chilling the dough overnight is recommended to prevent excessive spreading.

Preheat the oven to 325°F (163°C). Line large baking sheets with parchment paper or silicone baking mats. If the dough has chilled longer than 2 hours, allow it to rest at room temperature for approximately 15 minutes before shaping.

Portion the dough using a cookie scoop or measuring spoon: approximately 3 scant tablespoons (60 g) for extra-large cookies or 2 heaping tablespoons (50 g) for medium-large cookies. Roll each portion into a ball and shape it so it is taller than it is wide. Place cookies 3 inches apart on the prepared baking sheets.

Bake for 13–14 minutes, or until the edges are lightly browned and the centers appear soft. The cookies will continue to set as they cool.

Allow the cookies to cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely. Optional: press additional chocolate chips into the tops while the cookies are still warm.