Once the butter has cooled, pour chilled butter into a mixing bowl. I used my KitchenAid stand mixer, but you can also use a hand-mixer for this. Add brown sugar and white sugar and cream for 4-5 minutes, scraping the sides of the bowl often.
Add eggs and egg yolk, one at a time, beating for 30 seconds after each addition. After this step, you are done with the mixer. Stir in vanilla with a wooden spoon.
Add flour, baking soda, cornstarch and salt. Stir just until combined, avoid overmixing. Stir in chocolate chips.
Wrap cookie dough tightly with plastic wrap and let chill in the refrigerator for at least one hour. The dough can chill for up to 24-48 hours if you want to make ahead.
Preheat oven to 350 degrees. Line a baking sheet with parchment paper. Roll balls of cookie dough and place on cookie sheet. Bake for 11-12 minutes or until they are golden brown around the edges.
My oven has hot spots and yours may too so be sure to rotate the baking sheet once during baking. The baked cookies will look soft in the centers when you remove them from the oven. Allow to cool for 5 minutes on the cookie sheet. Enjoy!