In a large mixing bowl, combine the melted butter with the brown sugar and granulated sugar and whisk to combine and no dry sugar remains.
Whisk in the egg, egg yolk and vanilla until just combined.
Add the flour, baking soda, cornstarch and salt and fold in with a rubber spatula.
Fold in the chocolate chips.
Cover and refrigerate overnight. (min 3 hours - up to 4 days)
Once the dough is chilled, preheat the oven to 325 degrees.
Line baking sheets with parchment - set aside.
Scoop the cold dough into 3 tablespoon-sized balls and roll between your hands to smooth. The dough will be firm and crumble a little but should pull together with the warmth of your hands. Make the dough lightly taller and oblong, rather than round.
Place the dough on the prepared baking sheets, making sure the dough is standing on end so they look tall. This will help the cookies to bake thick.
You can press more chocolate chips around the outside of the dough before baking.
Bake for 14-16 minutes for cookies with 3 tbsp of dough. If making a bit smaller with 2 tbsp of dough - bake 12-13 minutes. They will look underbaked.
Let the cookies cool on the baking sheet for about 10-12 minutes.
Transfer to a cooling rack to cool completely before transferring to an airtight container.