Coating the skillet with vegetable oil inside and outside. I used a brownies mix set with a cast iron like this below.

To make a brownie, combine brownie mix, hazelnut chocolate spread, butter, and water into mixing bowl. Mix until completely blended.

Spread batter evenly onto skillet. Put a marshmallow on top the batter.

Remove skillet from oven and leave to cool for 5 minutes before serving.