Place the butter in a saucepan and melt over low heat. Increase heat to medium (do not leave your butter or it will burn) and swirl occasionally as the butter snaps and crackles.
Let the butter snap and pop (while stirring occasionally) and once this begins to slow, begin to stir and don't stop. The butte will be foamy so it will be difficult to tell once it has begun to brown, but once you begin to smell the unmistakably nutty aroma of browned butter, remove from heat (still stirring continuously) and pour into a heat-proof container (If you accidentally burn the butter, which you will know if you did, it will
burnt, then do not use it or you will have burnt-tasting cookies.).
Allow butter to cool for at least an hour and a half before proceeding.
Once butter has cooled, combine in KitchenAid (or with hand-mixer) with sugars, beating on medium-speed until well combined
Add eggs and egg yolk, one at a time, pausing to scrape down the sides and then beating well after each addition.
Add bourbon and vanilla. Stir.
In separate bowl, combine flour, cornstarch, baking soda and salt.
Gradually (with mixer on low-medium speed) add flour mixture to butter mixture, pausing periodically to scrape down sides of bowl.
Stir in chocolate chips/chunks, transfer to container with a lid and refrigerate overnight.
Once dough has refrigerated, preheat oven to 350F (177C) and line cookie sheets with parchment paper.
Scoop dough either into ice cream scoop or into 1/4 sized measuring cup and drop onto prepared cookie sheets (the dough is difficult to work with, I have to scoop out smaller pieces of dough at a time and press them into my ice cream scoop into a cohesive ball and
drop them onto the cookie sheet). Press down gently on balls to flatten slightly.
Bake 10-12 minutes.
Remove and allow to cool completely at least 10 minutes on cookie sheet before transferring to cooling rack to cool completely.