The crisp butty is the crisp sandwich spoken in its affectionate register. The word does most of the work: butty is the Northern and informal British name for a buttered-bread sandwich, and attaching it to a packet of crisps signals exactly what this is, a thing made at home from whatever is in the house, owned and named with the same fondness as the chip butty it sits beside. The construction could not be plainer. Crisps go on soft white bread spread thick with butter, the bread is folded over and pressed, and that is the whole sandwich. Calling it a butty rather than a crisp sandwich is not a different recipe. It is a different relationship to the same idea.
The craft is the press and the butter. The crisps bring all the texture and none of the fat, so the butter is structural: it grips the crumb, anchors the loose crisps so they do not all slide out the open side on the first bite, and supplies the salted richness a dry crisp on dry bread would lack. The press matters as much as the spread. A firm push down shatters the crisps in place, binding a heap of separate pieces into something that holds together for a few bites and producing the sound and the give that are the entire pleasure. The bread is deliberately soft and plain so it yields against the shards rather than competing with them, and it has to be eaten promptly, because the crunch the butty exists for is on a short clock once the crisps meet the butter and the moisture in the bread.
The variations are the crisp aisle and the regional vocabulary. Cheese and onion, salt and vinegar, and ready salted each change the seasoning while the structure stays fixed; the same sandwich answers to barm, cob, or bap as you move across the country, the local bread word riding on top of an idea everyone agrees on. Each keeps the crunch-against-soft-bread logic and deserves its own article rather than being crowded in here.