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

In a medium bowl, whisk the melted butter, brown sugar, and granulated sugar together until no lumps remain. Whisk in the egg and egg yolk until combined, then whisk in the vanilla extract. The mixture will be thin. Pour into dry ingredients and mix together with a large spoon or spatula. The dough will be very soft, thick, and shiny. Fold in the chocolate chips.

Cover the dough tightly and refrigerate for at least 2 hours or up to 3 days. It's recommended to chill the cookie dough overnight to prevent overspreading.

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

Using a cookie scoop or Tablespoon, scoop the chilled cookie dough, about 3 scant Tablespoons (about 2 ounces) for XL cookies or 2 heaping Tablespoons (about 1.75 ounces) for medium-large cookies. Shape into a tall ball and arrange 3 inches apart on prepared baking sheets.

Bake the cookies for 13–14 minutes until the edges are lightly browned. The centers will look very soft but will set as they cool.

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

Store tightly covered at room temperature for up to 1 week.