Place a 10-inch, seasoned cast-iron skillet on the middle rack in your oven and preheat the oven to 350 degrees while you prepare the brownie batter.

Whisk together the brownie mix and all-purpose flour in a large bowl until combined.

Beat the melted butter and eggs until well combined, about 2 minutes.

Fold in the chocolate chips until evenly distributed throughout the batter.

Carefully remove the hot skillet from the oven and pour the batter into the preheated skillet. Use a spatula or spreading knife to spread it out evenly.

Place the skillet in the preheated oven and bake for 20-25 minutes, or until the edges are set and a toothpick in the center comes out gooey but not doughy. It will continue to cook as it cools, so make sure you take it out long before a toothpick comes out clean.

Allow the skillet brownie to cool for 10-15 minutes before topping it with vanilla ice cream, optional, and serve warm.