In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.

Add the egg and egg yolk and mix until fully incorporated.

Add the flour, cornstarch, baking powder, baking soda, and salt. Mix until a soft dough forms. Fold in the chocolate chips.

Divide the dough into approximately 9 equal portions, about 120 grams each.

Roll each portion into a smooth ball and place them on a plate or baking sheet. Refrigerate the dough balls for at least 2 hours.

Once chilled, place the cookie dough balls onto the prepared baking sheets, leaving plenty of space between each cookie to allow for spreading. Preheat oven to 350 degrees F.

Bake for 13 to 15 minutes, or until the edges are lightly golden and the centers still appear slightly underbaked.

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