A beef paste sandwich is governed by a ratio, and the ratio runs the opposite way to almost every other sandwich. Here the filling is the thing you use least of. Cooked beef is cured, reduced, and pounded with fat and seasoning into a smooth, dark, intensely salty spread that keeps in a sealed jar at the back of a cupboard, and the entire design follows from that keeping. A paste that has to survive months unrefrigerated is concentrated to the point where a thin scrape across buttered bread is already a full-strength mouthful. Spread it like a filling and it is inedible. Spread it like a stain and it is a sandwich.
The craft is the scrape and the butter underneath it. Butter is not optional lubrication here; it is the carrier that drags the paste's salt across the slice and stops a dark smear reading as a smear. The bread is soft and plain because the paste brings no texture of its own and a crust with real chew would have nothing to chew against. There is no heat, no second layer, no acid: the sandwich is built to be assembled cold, cut thin, pressed, and left in a tin until lunch, and it is one of the few sandwiches that improves slightly for sitting, the paste working into the crumb rather than weeping out of it. Its whole logic is that a very small amount of something very strong can turn bread and butter into a meal, which is the same thrift instinct that runs the dripping slice and the sugar sandwich.
The variations are a row of jars rather than a row of recipes. Chicken paste is paler and milder, ham paste leans on its cure, crab and salmon paste carry the same potted logic into fish, and bloater paste takes it somewhere else entirely with smoked herring. Each is the same engineering met with a different protein, and each deserves its own article rather than being crowded in here.