First of all, make sure your ice cream maker insert has had a chance to freeze for a full 24 hours.
Heat the cream, half and half and marshmallows in a medium saucepan over medium heat. Stir until the marshmallows are melted into the cream. Remove from heat and stir in the vanilla and add the salt. Pour the mixture into a bowl, cover it and refrigerate overnight or until completely chilled.
Process the ice cream base in your ice cream maker according to your ice cream maker instructions (ours took about 30 minutes to turn into thick ice cream). At the end of processing, spoon in some generous spoonfuls of hot fudge. You just want to swirl it in there instead of getting the whole thing to turn into chocolate. Turn off the ice cream maker and scrape the ice cream into a freezer-safe container. You can swirl in more fudge if you want. Freeze until firm, then scoop and eat!