Add a slab of cold butter to an anko sando and you get the sweet-salty reading that Nagoya is known for. The structure is the same as the plain version, anko, the sweetened red bean paste, layered between thick slices of soft shokupan, but a cold, firm plank of butter is set against the paste so each bite carries both. This is what makes it its own thing rather than a variant footnote. The paste brings dense sweetness, the butter brings salt and fat and a cool, slow melt, and the bread holds the two apart just long enough for the contrast to register. Take away the butter and it is a different, simpler sandwich, which is exactly the point.
The craft is in the butter and the temperature. It wants good cultured or lightly salted butter, cut thick as a deliberate layer rather than smeared thin, so it stays distinct against the paste instead of disappearing into it. The whole sandwich is best kept cool so the butter holds its shape and gives that firm, melting bite rather than turning greasy. The shokupan should be fresh, thick, pillowy, and trimmed, and the anko, smooth or chunky, spread evenly so it meets the butter across the whole face of the sandwich. A good one is balanced, the salt of the butter lifting the sweetness so neither runs away. A poor one is butter so thin it is lost, or paste so heavy it buries the dairy, leaving just a sweet sandwich with the trick missing.
The plain anko sando, paste and bread alone, is its sibling and its baseline, and the difference between them is entirely the butter. From there the wider family includes fruit sandos on the same shokupan and the sealed an pan line of buns, and each of those deserves its own article rather than being crowded in here.