Marmite and butter names its partner in the title, and the butter is the point. Marmite is a thick, near-black yeast extract that is almost pure savoury salt and bitterness, with no fat and no body of its own, and butter is the ingredient it is built to be cut into rather than spread alongside. On bread, butter is laid down first and generously, and the Marmite is dragged across it in a thin film so the two combine into a single buttered-savoury layer rather than sitting as separate stripes. The defining logic is that the fat and the gentle dairy salt of the butter are what make the extract edible at all: butter rounds the sharp edge, distributes the intensity, and turns a salt-shock into a deep, rounded savoury note. This is the version that exists specifically to foreground that partnership.
The craft is the ratio between the two and how they are combined. Generous butter is structural here, not lubrication: the more butter relative to Marmite, the softer and more rounded the result, so the balance is set deliberately rather than by accident, with the Marmite kept to a scrape and the butter kept ample. Working the extract into soft butter on the slice, rather than laying it on cold and firm, gives an even mid-brown layer with no concentrated dark patches to ambush a bite. The bread is plain and soft so it yields to a filling that has no texture of its own, and a little extra salted butter is what stops the savouriness reading as one flat, bitter note. There is no heat and no second filling, and a sandwich made this way holds steady in a tin because the buttered crumb absorbs the extract rather than letting it bleed.
The variations move the same idea into different registers. The crustless afternoon-tea version trims it small and delicate; Marmite on toast lets heat thin the spread further on a single slice; Marmite and cheese adds a tempering richness on top of the fat; the bare Marmite sandwich strips the butter back toward the minimum. Each deserves its own article rather than being crowded in here.