An initial attempt with chocolate on a graham cracker to see if the fire was hot enough to melt anything at that distance. When it was, I added marshmallow but it turned into a mess because the chocolate ran everywhere before the marshmallow had a chance to roast.
I broke the marshmallow in half
and gave it a quarter turn as the side closest to the fire got toasted.
When it was ready, I added the chocolate and the top graham cracker. It turned out perfect, melted but not burned.
The only downside was the cracker got really hot (and as you can see, above, a little over-toasted) - I used to long-handled spatula to turn it, but I think next time I'd grab tongs (also with long handles) so I could get a good grip on the cracker.