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 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 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 baking sheets with parchment paper or silicone mats. If dough has chilled for longer than 2 hours, let it sit at room temperature for about 15 minutes.

Using a cookie scoop, scoop the chilled dough and roll into a ball, shaping it taller rather than wide. Arrange the cookies 3 inches apart on the baking sheets.

Bake for 13–14 minutes or until the edges are 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.