Marmite and butter in the tea register is the same thin scrape of yeast extract translated into the crustless finger sandwich. The defining facts are size and trim, not the spread. Marmite is a thick, near-black extract that is almost pure savoury salt, and here it goes on soft white bread that has been buttered to the edges, scraped to translucence, pressed, had its crusts cut away, and been sliced into small fingers or triangles meant to be eaten in two bites without a plate. The trimming is the design, exactly as it is for cucumber or egg on the tea tray: a crust would resist a filling that has no texture of its own, and on this scale the point is that nothing should resist. It is the polite, delicate reading of a famously divisive spread.
The craft is restraint stacked on restraint. The Marmite is kept lighter here than even the everyday sandwich, because a finger of bread is small and a heavy scrape would dominate a portion meant to be one of several savouries on a stand. Butter to the edges does the structural work, carrying the salt evenly across a thin slice and waterproofing the crumb so the sandwich survives the half hour between assembly and the table. The bread is the softest and plainest possible, sliced thin so the buttered savoury layer is the larger part of each bite, and the squares are cut clean with a sharp knife so the edges stay neat on the plate. There is no heat and no second filling; the whole effect rests on a faint, deep savouriness against plain bread, sized to be eaten without disturbing a conversation.
The variations are the rest of the tea canon and the everyday forms of the same spread. The standard Marmite and butter sandwich keeps the crusts and a heavier hand; Marmite on toast moves it open-faced and lets heat thin it; Marmite and cheese adds a tempering richness; cucumber and smoked salmon sit beside it on the same crustless logic. Each deserves its own article rather than being crowded in here.