What defines the Panino con Prosciutto di Parma is the cure that took the longest and added the least. The leg is rubbed with salt and nothing else, no pepper, no smoke, no sugar, then hung in the dry air drawn off the Apennine valleys around Parma through a long slow cure while the fat turns sweet and the lean goes deep red and nutty. There is no seasoning to taste in it because there was no seasoning put on it: what reaches the bread is pork that has spent a long Emilian patience becoming concentrated. The single most important thing the salumiere does at the counter is slice it to translucence, so thin that the slice drapes and the light comes through it, and that one act decides whether the sandwich works.
The craft is the slice and the bread it lies on. Parma is fatty and gentle, so it is cut almost to nothing and laid in loose, airy folds rather than flat sheets, which lets the fat melt against the tongue instead of reading as a slab. The sweetness is the whole argument, so the bread is kept neutral: a crisp-shelled rosetta or a piece of unsalted Tuscan loaf, plain enough to carry the ham without arguing with it. Butter appears rarely and only thin, where a very lean part of the leg meets a dry crust and needs a bridge. Assembled, it is eaten close to room temperature, never fridge-cold, because cold mutes the fat that is the entire point of this particular cure.
The variations are the other raw-cured legs of Italy, and each is a separate ham rather than a note here. The Friulian prosciutto di San Daniele, pressed flat with the trotter left on and a touch sweeter still; the saltier, peppered prosciutto toscano eaten against unsalted bread; the robust mountain prosciutto di Norcia; the lightly beech-smoked prosciutto di Sauris; the lean, dark, gamey prosciutto di cinghiale made from wild boar. Each is its own salt, its own air, its own bread match, and each deserves its own article rather than being crowded in here.