Anko sando takes a traditional Japanese sweet and reads it as a sandwich. Two slices of shokupan, the soft, square white milk bread cut in thick slices, are spread with a generous layer of anko, the sweetened red bean paste. There is nothing else in it. Unlike an pan, where the paste is sealed inside a baked bun, here the bread is sliced loaf bread and the paste is laid between flat slices like a filling, which changes everything about how it eats. The two parts still need each other for the same reason any sweet sandwich does, but the shokupan is doing different work than an enriched roll, cool and tender and clean, a neutral frame for a sweet that usually arrives in a lacquered box.
The craft is mostly in the shokupan and the spread. The bread should be fresh and pillowy, cut thick enough to stand up to a heavy layer without tearing, with the crusts trimmed so the bite is all soft crumb and paste. The anko, smooth koshian or chunky tsubuan depending on the maker, is spread edge to edge and corner to corner at an even thickness, so every part of the sandwich tastes the same and there are no dry margins of bare bread. A good one is cut cleanly to show a flat, level seam of dark paste against white crumb, the way a fruit sando shows its fruit. A sloppy one is thin in the middle and thick at one edge, or made on stale bread that fights the paste instead of yielding to it.
The obvious companion is the anko and butter version, where a slab of cold butter joins the paste for a sweet-salty Nagoya reading, a real shift in flavor and intent rather than a small tweak. From there the family runs to fruit sandos on the same shokupan and to the baked an pan line, and each of those deserves its own article rather than being crowded in here.