Preheat oven to 350 degrees. You will need a 6 inch cast iron skillet for this recipe.
In a mixing bowl combine the melted butter and both sugars. Use a large spoon to mix the ingredients together until combined. Add in the egg yolk and vanilla extract, continue to stir everything together. Add in the flour, baking soda & salt. Mix together so that a cookie dough forms. Cookie dough will be dark, slightly wet and a little on the thinner side. Fold in the chocolate chips. Transfer the cookie dough to the cast iron skillet and spread the dough out so it evenly covers the bottom of the skillet.
Bake in the oven for 18 to 22 minutes. When cookie skillet is done, remove from the oven and allow to cool for 5 to 10 minutes. Sprinkle flaked sea salt on top if desired. Top with your favorite flavor of ice cream, hot fudge, whipped cream, etc. Serve immediately.