The anchovy paste sandwich is governed by a single number: how thinly the paste is spread. This is a jarred preparation, anchovies cured and pounded with butter and spices into a smooth, dark, intensely salty spread that keeps in the cupboard, and the entire skill of the sandwich is restraint. Spread to the thickness of jam it is inedible, a wall of salt and fish. Spread to the thinnest possible film, dragged across buttered bread so it tints rather than coats, it becomes a savoury seasoning for bread and butter. The paste is not the filling. It is closer to a condiment used in the smallest quantity that still registers.
The craft is the ratio between three things that are all doing different jobs. The butter goes on first and goes on properly, because it is the carrier: it spreads the salt evenly across the crumb and stops the concentrated paste reading as a smear in one spot and nothing in the next. The paste is then worked over the butter in a quantity that looks insufficient and is correct. The bread is soft, plain, and thin, with the crusts often taken off, because a filling with no texture of its own gains nothing from a chewy crust and everything from being delicate. Cut small and pressed, it keeps for hours, which is the point of starting from a jar in the first place: a sandwich that survives a lunch tin and asks nothing of a kitchen.
The form sits within a wider larder of pounded protein spreads, beef and chicken and salmon and crab pastes among them, each carrying a different cure but obeying the same thin-spread logic. The open-faced anchovy treatment, where the fish is mashed onto toast rather than sealed thin between soft slices, is a genuinely different sandwich and deserves its own article rather than being crowded in here.