Preheat oven to 350°F. Grease the interior of a 10-inch oven-safe skillet. For best results, use a cast iron skillet.

To make the brownies, add melted butter and cocoa powder to a large bowl. Whisk until cocoa powder is completely dissolved and the batter is glossy and smooth. Add in sugar and whisk until completely dissolved. Add in eggs and vanilla and whisk until completely mixed in. Switching to a spatula, add in flour and salt. Stir batter gently only until flour is incorporated and no white streaks remain but be careful not to overmix. Spread batter into the bottom of the cast iron skillet.

To make the cookies, add melted butter, white sugar, and brown sugar to a large bowl. Whisk until sugars are completely mixed in and batter looks light and fluffy. Add in vanilla and egg. Whisk until mixed in. Switching to a spatula, add in flour, baking soda and salt. Stir batter gently only until flour is incorporated and no white streaks remain but be careful not to overmix. Mix in 2/3 cup of chocolate chips.

Pour the cookie dough on top of the brownie mix and spread out evenly but make sure to leave a 1-inch brownie crust around the edges. Sprinkle remaining 1/3 cup of chocolate chips over the entire surface.

Place skillet into the bottom half of the oven (this will prevent your cookie layer from turning too brown). Bake for about 25-30 minutes or until brownie layer is completely cooked and cookie layer is golden brown on top but not quite set inside. If you wish for a fully cooked cookie layer you can bake until cookie layer is cooked as well.