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
The Chocolate Marshmellow is like an adult version of the Hostess Cupcake - with a bittersweet chocolate ganache and billowy marshmellow filling (that didn't taste like the thick gooey marshmellow creme that comes in a jar for those of you who are wondering - it was much more refined than that.) But I have to say that my favorite is still the lemon coconut. Mmm, I wish I had one right now!
P.S. Isn't their flavor chart adorable? You can pick one up at their stores so you can keep it up and tempt yourself daily. (I had to take mine down for that very reason!)