Melt the butter in a light-colored saucepan over medium heat, whisking constantly until the butter begins to foam. When the butter under the foam is golden and smells sweet with a nutty aroma, remove the pan from the heat and transfer to a bowl to cool for 10-15 minutes.
When the butter has cooled, add the sugar and brown sugar. Mix with the paddle attachment on medium speed for 2-3 minutes until well combined.
Add eggs + egg yolk one at a time, blending well after each addition. Add the vanilla and mix to combine.
Add flour, cornstarch, baking soda, baking powder, and salt. Mix for a few seconds, then add the chocolate chips. Mix until just combined.
Use a large cookie scoop to scoop cookie dough balls. Place in an airtight container, cover, and chill for at least 2 hours, up to 1 day.
When ready to bake, preheat the oven to 350 degrees F. Line two baking sheets with parchment.
Roll 6 cookie dough balls and place onto a prepared baking sheet. Bake for 10-12 minutes, or until the edges are golden brown and the centers look just underdone. Cool on the baking sheet for 2 minutes before transferring to a wire rack.
Enjoy right away or store in an airtight container for 2-3 days, or freeze for up to 2 months.
