In a 8-10" oven-safe skillet (such as cast iron), heat cream over medium heat to steaming. It should be hot but not boiling.
Add chocolate and whisk until melted. Remove immediately from heat.
Arrange marshmallows on top of melted chocolate.
Bake at 350°F for about 3-5 minutes. Then turn the broiler to high and continue cooking an additional 2 minutes, or until marshmallows are lightly toasted.
Video
Notes
Boiling cream can separate and curdle, so don't bring the cream to an actual boil. Just heat it to so it's hot or steaming.
Watch the broiler carefully, as the marshmallows can overbrown in a matter of a few seconds (Trust me on this. I've been known to get distracted and over-toast the marshmallows on more than one occasion...)
Use high quality chocolate or chocolate chips.
You can also use mini marshmallows. Just cover the chocolate with them. and follow the same baking and broiling instructions.