Marmite and peanut butter is the combination that sets a sweet, nutty richness directly against a sharp savoury salt, and the interest of the sandwich is entirely in that opposition. Marmite is a thick, near-black yeast extract that is almost pure glutamate and bitterness; peanut butter is fatty, faintly sweet, mild, and heavy-bodied. They have almost nothing in common, which is the point: the peanut butter's oil and gentle sweetness blunt the extract's salt-shock and give it a soft, rounded base to sit on, while the Marmite cuts the cloying flatness that a thick peanut layer has on its own. Each fixes the other's main fault. The defining decision is keeping the Marmite to a thin scrape so it seasons the peanut butter rather than fighting it.
The craft is the ratio and which layer carries which. Peanut butter goes on as the substantial layer, spread thick enough to be the body of the sandwich, and the Marmite is laid on in a thin film, often scraped onto the opposite slice or worked lightly over the peanut butter so the two meet rather than sitting in a hard seam. Smooth peanut butter spreads evenly against the extract; the crunchy style adds a textural pop that an all-soft savoury-sweet filling otherwise lacks. The bread is plain and soft because both components are dense and assertive and a chewy crust would only add a third argument. There is no heat and no second filling, and the build holds reasonably well in a tin because the peanut oil seals the crumb against the extract.
The variations stay close to the savoury-sweet axis. Marmite and butter strips out the sweetness and runs fat against salt alone; the plain Marmite sandwich lives on the scrape with no foil at all; peanut butter with jam or banana is the same nut base met with a clean sweetness instead of a savoury one; Marmite on toast moves the open-faced reading. Each deserves its own article rather than being crowded in here.