Place 6" cast iron skillet in the oven. Preheat oven to 450°F with the pan inside.
Once the oven is preheated, heat the heavy cream in a microwave safe bowl until it's steaming hot, about 30 seconds.
Remove the skillet from the oven. Add chocolate chips and salt to the hot skillet. Pour hot heavy cream over top. The heat from the skillet and the heavy cream will melt the chocolate chips. Stir chocolate chips with a spatula until mixture is smooth. Spread mocha mixture into an even layer. Add marshmallows on top in an even layer.
Bake 7-8 minutes, until marshmallows are toasted and golden brown. Allow to cool for 5 minutes.
Serve with graham crackers. I dip for my daughter, as the skillet is very hot!