Preheat oven to 350°F and grease a 10-inch cast iron skillet with butter.
Whisk together melted butter and brown sugar, then stir in egg and vanilla.
Mix in flour, baking soda, and salt until just combined, then fold in chocolate chips and crushed graham crackers.
Spread dough evenly into the skillet and press mini marshmallows into the top.
Bake for 20 to 22 minutes until edges are set and golden but the center is still slightly soft.
Serve warm directly from the skillet with scoops of vanilla ice cream.