In Lancashire and across the Northwest the chip sandwich is a chip barm, and the word is not interchangeable with any other. A barm is a soft, flattish white roll with an open, springy crumb and a pale floured top, the name pointing at the leaven that lifts it. It sits lower and looser than a tall bap, its crumb full of small irregular holes that flatten under a hand and then spring part of the way back. The barm is engineered to be squashed, and a barm that resists the press has been baked wrong. That is exactly the behaviour a pile of chip-shop chips wants over it. The chips, the butter, and the press are the constant in every chip sandwich; the barm is what makes this the Northwest's reading of it.
The craft is the same fat-and-heat discipline the format always demands, met by an open crumb that absorbs differently. The chips go in straight from the fryer because the sauce of the thing is butter melting on hot potato into a slick that glues bread to filling, and a cooled chip will not produce it. The barm is buttered to the edges while still cool so that emulsion forms inside the roll rather than passing straight through the soft floured top, and the open lower crumb takes a measured amount of fat and steam into its cells and weights into something dense and savoury while the lid stays dry and matt. The chips are laid flat rather than heaped so the barm closes cleanly, then the roll is pressed firmly so its loose, holey crumb collapses around the chips and binds the loose pile into something that holds for a few bites. Salt and vinegar from the chip shop, or a stripe of brown sauce, goes on before closing so it stays inside.
Out of Lancashire the identical sandwich changes its word with the map. It is a bap across much of England, a cob through the East Midlands, a plain chip butty wherever the bread is left unspecified and soft white is assumed. Drown it in chip-shop curry sauce, gravy, or mushy peas and the snack becomes a meal. Those deserve their own articles rather than being crowded in here.