Preheat the oven to 350° F and grease a 10 or 12-inch cast-iron skillet with butter.
In a medium mixing bowl, whisk together the flour, baking powder, baking soda, salt, and cornstarch. Set the bowl aside.
In a stand mixer fitted with the paddle attachment, cream together unsalted butter and cream cheese until smooth. Add the granulated sugar and light brown sugar. Scrape down the sides and bottom of the bowl as needed. Then add the vanilla extract and beat until combined.
Reduce speed to low and gradually beat in flour mixture until combined.
Beat in chocolate chips until evenly distributed.
Using a rubber spatula or wooden spoon, spread the cookie dough evenly into prepared skillet. Then sprinkle it with the sea salt and more chocolate chip on top, if desired.
Bake the cookie skillet for 30 to 35 minutes, or until it’s puffed, barely cooked in the center, and browned on the sides. If you want the cookie skillet less chewy you can bake it a little longer, around 40 minutes.
Remove it from the oven and allow it to cool for 10 minutes.
You can scoop/slice and serve or serve topped with ice cream and a whole bunch of spoons so everyone can dig in together.