Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper.
Cream the softened butter and brown sugar in a large bowl for 2-3 minutes until light and fluffy.
Mix in the egg, egg yolk, and vanilla extract until well combined and smooth.
Whisk the flour, cornstarch, baking soda, and salt in a separate medium bowl.
Combine the dry ingredients into the wet mixture slowly, mixing only until no flour streaks remain.
Scoop the dough into 1.5-tablespoon balls and place them on the baking sheet, 2 inches apart.
Bake for 9-11 minutes. The edges should be set but the centers soft. Do not overbake.
Cool on the baking sheet for 5 minutes before transferring to a wire rack.
