I questioned nuking in a plastic bag too. I have stopped putting ANY plastic in the microwave and I even cover things with cheapie paper plates for less splatters. I have, however, used the recipe for omelettes boiled in ziploc bags. My grandkids will love this one! And I'll probably use chocolate syrup.
From EatingWell: January/February 2009
You don't have to sit around a fire to enjoy S'mores. Just be sure to watch them carefully under the broiler—the marshmallows can go from perfectly toasted to charcoal in a hurry.
8 Reviews for Indoor S'Mores
It might be better to microwave it in glass. It would be a great idea but I'm pretty sure that heating plastic bags releases unsafe chemicals into your food. This looks delicious though :)
OR, microwave the chocolate in one corner of a ziplock snack bag and snip off the corner to make a mess-free applicator.