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.

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

Using a cookie scoop or Tablespoon measuring spoon, scoop the chilled cookie dough, about 3 scant Tablespoons for XL cookies or 2 heaping Tablespoons for medium-large cookies. Roll into a ball and shape so that it’s taller rather than wide. Arrange the cookies 3 inches apart on the prepared baking sheets.

Bake the cookies for 13–14 minutes or until the edges are very lightly browned. Cool on the baking sheet for 10 minutes before transferring to a cooling rack.

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