Sweetness changes everything about how a hot sando behaves under the press. The anko butter version takes the sealed-pocket format from the baseline pressed sandwich and fills it with anko, the sweetened red bean paste that runs through Japanese confectionery, plus a slab of cold butter. Inside the closed iron, those two ingredients do something a savory filling cannot: the bean paste warms into a loose, jammy state while the butter melts entirely and runs through it, so the parcel that emerges holds a hot, glossy, faintly salted sweet center inside a toasted shell. It reads less like a sandwich and more like a pressed pastry that happens to use sliced bread.
The technique question here is about heat and quantity rather than welding alone. Anko comes in two registers, the smooth koshian and the chunkier tsubuan, and either works, but the paste should be spread to a margin like any hot sando filling so the bread can still seal at the crusts. The butter is the variable that separates a good one from a greasy one. Too much and it pools out of the crimped edge and fries the outside in its own fat; too little and the center stays dense and pasty instead of turning into something that flows. Done with judgment, the press toasts the shokupan to a sweet, lacquered crust while the interior goes molten, and the contrast of crisp bread against warm bean paste with a thread of salted fat is the entire appeal. A poor version is easy to spot: a cold, claggy core where the heat never penetrated the dense paste, or scorched bread because the cook compensated by leaving it on too long. The bean paste should be hot all the way through rather than warm only at the edges.
This belongs to the broader an-pan and anko lineage as much as to the pressed-sandwich family, which is why its closest relative in the catalog sits among the sweet breads rather than the savory sandos. Common adjustments stay within that sweet frame: a thicker butter plank for people who want it richer, a dusting of nothing because it needs none, or a pairing with bitter coffee that exists specifically to cut the sweetness. There is also a fruit-forward cousin that swaps in mochi or a layer of fresh fruit, and that combination is enough of a distinct experience that it deserves its own article rather than being crowded in here.