The panino con Parmigiano-Reggiano is a sandwich about a crystalline bite. Parmigiano-Reggiano is the hard cow's-milk wheel of Emilia, aged long enough that the paste turns dry, granular, and shot through with the white crystalline specks of crystallised amino acids that crunch and dissolve on the tongue. The defining move of this sandwich is the format of the cheese: it is broken or shaved into irregular shards rather than sliced flat, often with a thread of aceto balsamico over them, so each piece gives a discrete burst of salt, savour, and that crystal grit instead of a uniform layer. The bread is plain on purpose, present only to carry the shards, because a great wheel at its age is loud enough and a strong loaf would only blur its edges.
The craft is the age, the break, and the balance of vinegar. A younger Parmigiano around twelve months is milder and more elastic; a twenty-four or thirty-six month wheel is drier, more concentrated, and more crystalline, and which one goes in changes the sandwich entirely. The cheese is not cut with a knife but levered apart with the short almond-shaped coltello so it fractures along its own grain into rough shards, the shape that gives the bite its texture. A few drops of aged aceto balsamico di Modena answer the salt with a dark sweetness and a little acid; used sparingly it lifts the cheese, poured heavily it drowns it. Nothing else belongs. The bread carries; the shard, the crystal, and the drop of vinegar are the whole composition, assembled simply because the components are doing all the work.
The named directions are mostly about age and partner. There is the build with fresh figs or pear that answers the salt with fruit sweetness rather than vinegar, the version with a drizzle of honey, and the pairing with walnuts for an oily, tannic counter. The Grana Padano sandwich is a near cousin from a different denomination and reads softer and less crystalline; it is not this. Each of these is its own preparation, and each deserves its own article rather than being crowded in here.