• Preheat oven to 350F and lightly grease a 10" cast iron pan with butter. Set aside.

Brownie Layer

• Combine butter and chocolate chips in a large, microwave-safe bowl and heat in 20 second increments (stirring well in between) until chocolate and butter are melted and well-combined.
• Add cocoa powder and stir well, then stir in sugars.
• Add eggs, egg yolk, vanilla extract, and salt, and stir until very well combined.
• Gradually add flour and stir until incorporated. Fold in chocolate chips. Set aside while you prepare your cookie layer.

Cookie Layer

• Using an electric mixer, cream together butter and sugars until well-combined.
• Add egg, egg yolk, and vanilla extract and stir well.
• In a separate, medium-sized bowl, whisk together flour, cornstarch, baking powder, and salt.
• Gradually add flour mixture to butter mixture and stir until combined. Fold in chocolate chips.
• Alternate dropping brownie batter and cookie dough by heaping spoonfuls into prepared cast iron pan. Use a knife to swirl the batter evenly.
• Transfer to oven and bake on 350F for 40-50 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs (not wet batter). If you notice that the top is beginning to turn brown but the center is still not cooked, cover the pan with aluminum foil so that the top will not over-cook.
• Allow to cool a bit after baking, but I personally believe this skillet brownie cookie is best when served warm, and even better if topped with vanilla ice cream!