The Panino al Tartufo is a sandwich governed entirely by quantity and restraint, because its defining ingredient is one that punishes excess. The truffle, in central Italy most often the black tartufo nero di Norcia, is intensely aromatic and short-lived, a flavour that fills a room from a shaving and turns muddy and bitter the moment there is too much of it. The panino is therefore not about building anything; it is about how little to use and what quiet thing to set it against. A few shavings of fresh truffle, or a careful smear of a good truffle paste, on a plain bread with a fat that carries aroma, and the discipline to stop there.
The craft is measure above all else. Truffle is volatile: its aroma is at its fullest the moment it is cut and fades steadily after, so a truffle panino should be assembled close to when it is eaten rather than held. The quantity is kept deliberately small, because the line between haunting and overwhelming is narrow and crossing it ruins the sandwich rather than improving it. The truffle wants a carrier of fat to bloom into, which is why it is so often met with a soft cheese, a film of butter, or a barely warmed egg rather than a lean meat that gives it nothing to hold onto. The bread is plain and the warmth gentle, because heat lifts the aroma but high heat scorches it, and a loud loaf would only bury a flavour that works by suggestion.
The variations are the contemporary paninoteca and the gourmet counter working the same volatile ingredient, and each is its own preparation rather than a footnote here. The fresh-shaved seasonal version, the salsa tartufata paste reading for when the season is out, the truffle-and-cheese build, the truffle laid over egg: each strikes its own balance of quantity against carrier. Each of those earns its own treatment, and each deserves its own article rather than being crowded in here.