Preheat the oven to 350 degrees.
In a bowl, beat together the ROOM TEMPERATURE butter (if it is melted at all these cookies will be flat. Don't microwave it) with the brown and white sugar until creamy.
Add in the vanilla extract, egg yolk, baking soda, and cornstarch. (Add 1/8 teaspoon salt here if using unsalted butter)
Beat until just combined.
Add in the flour and chocolate chips and beat until just combined.
Form 3 evenly sized, large and tall balls of cookie dough
Place the cookie sheet with cookie balls in the freezer for 15 minutes.
Remove and immediately bake for 9-14 minutes or until very lightly browned around the edges. (Cookies are always best slightly under-baked!)
If desired (pretty appearance + more chocolate!), press a few additional chocolate chips into the tops of the cookies right after pulling them out of the oven.