the secret is you don't put the marshmallows on until the end... You cook it and caramelize the nutella first, (along with bananas and mascarpone, then at the end, pull pizza out and while it's still on the peel, over the sink, dump a bag of marshmallows on it. It should take half a bag or so.. then return it and keep it on the peel, holding it up to the top of the dome where its insanely hot, it takes about 20 seconds for them to toast and burn in nicely.
Afterwards, it has to set up for at least 10-15 minutes. That's the hard part. (the wait)...