Scraping a jar out at the end of the morning bake and thinking about marshmallow creme, which started as a way to make sugar go further in wartime — cooked up around Somerville and carried door to door before anybody put it in a jar on a shelf. The sandwich name came much later, 1960, out of an ad.
Anyway, the physical fact of it. Creme warm is a disaster: it goes slack and stringy and drags the crumb right off soft white bread, so you get grey pits and a torn slice. Keep the jar cool, spread it cold, and lay peanut butter on both slices first so the bread's sealed and the fluff sits on fat instead of sinking in. I learned that the loud way in a hot kitchen in August, with a knife full of lint.
The sandwich was in print long before anybody called it a fluffernutter — a 1918 promotional booklet for the creme had it as the Liberty Sandwich (wartime, so of course it was). Took another forty-odd years for the better name to turn up.
Rosa, the gym bag test is brutal on that sandwich. One of my old teammates packed a fluffernutter for a doubleheader, 90-some degrees, and by the fourth inning it was one slice, one unit, no architecture left. Peanut butter on both sides would've saved him.