Preheat oven to 325°F
Lightly grease oven-safe, 10" cast iron skillet (you must use an oven safe skillet)
In mixer bowl with paddle attachment, beat the butter smooth and creamy
Add granulated sugar & brown sugar and beat on medium-high until light & fluffy
Beat in eggs, vanilla & milk on high speed until well mixed
Scrape down the sides and bottom of the bowl as needed if not using a scraper paddle attachment
Add flour, cornstarch, baking soda and table salt on low speed until combined
Add the chocolate chips & mix on low
Transfer 1/2 the cookie dough into prepared skillet & press down into an even layer
Add 1/2 the chocolate bar, caramel pieces & sea salt
Top with remaining dough, pressing down again & then with remaining chocolate candy bar, caramel & salt
Bake for 27-30 minutes or until browned on the sides and lightly brown on top
If you like it a little more cooked through, bake for 35-40 minutes.
Allow the cookie to cool slightly in the pan on a wire rack before serving