The panino con casu marzu sits at the far end of the Italian cheese shelf, and the honest way to describe it is not to flinch from what it is. Casu marzu is a Sardinian sheep's-milk pecorino deliberately taken past ripeness: cheese flies are allowed to lay in it, and the larvae break the fat down into a soft, weeping, intensely pungent paste with an ammonia-sharp edge. The maggots are alive in the cheese when it is eaten. It cannot be sold legally and is made and shared informally on the island, which means the sandwich is less a menu item than a thing that exists, and treating it as the logical extreme of the same impulse that produces every strong cured cheese is the only sane framing.
The craft, such as it is, is moisture and restraint, because this is the wettest, most aggressive thing anyone would put on bread. The cheese has half-liquefied, so it is spread thin like a fierce paste rather than sliced, and it is laid on pane carasau or another sturdy Sardinian bread that can carry a slick of something this far gone without dissolving. A very little goes a long way: the ammonia and the salt are overwhelming in quantity, so the bread is doing real work as a brake on the intensity rather than as a neutral carrier. Nothing is added that would compete; there is nothing on the island's table loud enough to. It is eaten as it is, by people who have grown up with it, and described plainly rather than sensationally is the most respectful thing this catalog can do with it.
The variations are not really recipes but degrees and relatives. There is the riper and the less-ripe stage, and there is the broader Sardinian pecorino and fiore sardo tradition from which this is the deliberate overshoot. Each of those is a separate cheese and a separate sandwich on its own terms, and each deserves its own article rather than being crowded in here.