Preheat the oven to 350 degrees Fahrenheit and lightly grease a 10" cast iron pan with butter or non-stick cooking spray. Set aside.
Combine ¾ cup unsalted butter and ½ cup semisweet chocolate chips in a large, microwave-safe bowl. Heat in 20-second increments stirring in between until melted and combined.
Add ½ cup natural cocoa powder, ¾ cup granulated sugar, and ¾ cup packed brown sugar. Stir to combine.
Add 2 large eggs, 1 large egg yolk, 1 teaspoon vanilla extract, and ½ teaspoon salt. Whisk to combine.
Gradually add 1 cup all-purpose flour and stir until incorporated. Fold in ½ cup semisweet chocolate chips. Set aside while you prepare your cookie layer.
Cookie Layer
Using an electric hand mixer, cream together ¾ cup softened unsalted butter and ¾ cup packed brown sugar until well combined.
Add 1 large egg, 1 large egg yolk, and 1 teaspoon vanilla extract then stir well.
In a separate, medium-sized bowl, whisk together 2 cups all-purpose, 1 ½ teaspoon cornstarch, ½ teaspoon baking powder, and ¾ teaspoon salt.
Gradually add the flour mixture to the butter mixture and stir until combined. Fold in 1 cup semisweet chocolate chips.
Assemble and Bake
Alternate dropping the brownie batter and cookie dough by the spoonfuls into the prepared cast iron pan. Use a knife to swirl the batter evenly.
Bake for 40-50 minutes or until a toothpick inserted in the center comes out clean. If you notice that the top is beginning to turn brown too quickly then cover the pan with aluminum foil so that the top will not overcook.
Cool for a few minutes before enjoying then serve with vanilla ice cream on top!