The Marmite sandwich is a sandwich whose entire craft is quantity. Marmite is a thick, near-black yeast extract so concentrated in salt and glutamate that a teaspoon is far more than a slice of bread can carry, and the difference between a good Marmite sandwich and an inedible one is measured in grams of spread. Scrape it thin, almost to translucence, so the bread shows brown through it rather than black, and it reads as a deep, savoury, faintly bitter hum under buttered bread. Lay it on like jam and it overwhelms everything, a salt-shock that no amount of bread will rescue. That single decision, how little to use, is the whole sandwich, which is why it divides people so sharply: most of the hatred is really a verdict on a slice that was spread too heavily.
The craft is the scrape and the butter beneath it. Butter goes on first and to the edges, soft enough that the Marmite can be dragged across it in a thin film rather than gouged into the crumb, and it does two jobs at once: it carries the salt evenly across the slice instead of letting it sit in a concentrated stripe, and its fat blunts the extract's sharp edge so the savouriness lands rounder. The bread is soft and plain on purpose, since the spread brings no texture of its own and a crust with real chew would have nothing to chew against. There is no heat and no second filling. The sandwich is assembled cold, pressed, and cut, and a Marmite sandwich made with restraint is one of the few that holds well in a tin, the extract working into the buttered crumb rather than weeping out of it.
The variations are mostly attempts to give the bare spread a counterweight it does not have on its own. Marmite and butter leans harder on the fat as a foil; Marmite and cheese sets a tempering richness against the salt; Marmite and peanut butter runs a sweet, nutty body against the savoury edge; Marmite on toast moves the same scrape onto a single crisp slice and lets the heat thin it further. Each deserves its own article rather than being crowded in here.