Whisk the flour, baking soda, cornstarch, and salt together in a medium bowl. Set aside.
Beat the softened butter with the brown sugar and granulated sugar for about 2 minutes, until light and fluffy.
Beat in the egg, egg yolk, and vanilla extract until fully combined.
Add the dry ingredients and stir just until no streaks of flour remain.
Fold in the chocolate chips by hand.
Cover and refrigerate the dough for at least 30 minutes.
Preheat the oven to 350°F (175°C). Scoop the dough into 1.5-tablespoon balls, spaced 2 inches apart on a parchment-lined baking sheet.
Bake for 10 to 12 minutes, until edges are lightly golden and centers still look slightly underbaked.
Rest on the baking sheet for 5 minutes before transferring to a wire rack.
