Add butter to a large bowl and microwave until melted (about 30-40 seconds). Stir in both sugars, vanilla and eggs to the melted butter. Fold in the flour, salt, baking soda and cornstarch just until combined. Do not over mix. Fold in the chocolate chips. Spread 1/2 of the batter into a cast iron skillet in an even layer making sure to cover the bottom. No need to grease the pan. (Cast iron pans should already be seasoned.) I used a 12 inch skillet. 8-12 inch skillets would work for this recipe. Spread the hazelnut spread ontop of the cookie layer. (This can be hard to do, don\\\'t worry if a little mixes with the cookie) Add the rest of the cookie batter ontop of the hazelnut spread and smooth it out to cover the hazelnut spread. Bake for 25 minutes or until golden brown. It may look a bit giggly still but I like to under bake mine a bit. It continues to cook as it cools. If you prefer bake for 5 additional minutes. Cool for 20-30 minutes before serving. Cut into slices and serve warm with ice cream if desired!