In the bowl of a stand mixer with the whisk attachment on, cream together the butter, melted chocolate, and melted marshmallow until mostly smooth. Scrape down the sides with a spatula as necessary.
Cream in the vanilla and the cream.
Then slowly add the powdered sugar one cup at a time. Once all the sugar is added, whisk on high for about two minutes.
Use immediately, or keep refrigerated until ready to use. Make sure to let it sit out before frosting cupcakes or a cake to make it easier to spread.