Preheat oven to 350° F. Grease a 12-inch cast iron skillet and set aside.
In a large bowl, combine melted butter, brown sugar, and granulated sugar. Stir until smooth. Add the egg, egg yolk, and vanilla and mix until combined.
Add the flour, cornstarch, baking soda, and salt. Stir until flour disappears. Stir in the chocolate chips and sprinkles, if using.
Spread batter evenly in prepared cast iron skillet. Sprinkle the top with a little sea salt, if desired. Bake for 20-22 minutes or until golden brown. Don't over bake, you want the cookie to be slightly gooey. Remove from oven and let cool on a wire cooling rack for 10-15 minutes. Serve with ice cream, if desired.
Note-you can use a 10-inch skillet or pie plate. The cookie will be a thicker so you will have to bake it a little longer.