I'm sorry that this isn't a recipe for Chocolate Marshmellow Cupcakes...but I can tell you where to get one! I've finally gotten around to writing up my post on the new (or newer - not sure how long they've had them now) cupcake flavor at Sprinkles.
When I saw the Chocolate Marshmellow flavor listed on their website I begged my hubby to take me for a Friday date night. We each picked out two flavors as you can see. :D