The pie barm is the Wigan pie in a barm cake, and it is the most committed carbohydrate-on-carbohydrate sandwich in Britain. A whole meat and potato pie, shortcrust top and bottom, is put inside a split and buttered barm, the soft floured roll that Wigan and the wider Northwest use as their bread word. That is the entire build, and its defining fact is that it stacks three starches in a row: the pie's lid, the pie's base, and the barm around both. It is not an accident of thrift or a way of stretching a small filling. It is a deliberate, locally proud combination, eaten exactly as intended, and the redundancy is the identity rather than a flaw to apologise for.
The craft is the same logic the chip butty runs on, taken to its limit. The barm is soft and plain because the pie brings the structure, the seasoning, and the gravy-rich filling, and a bread with real chew would only add a fourth layer of resistance to something already firm and dense. Butter on the barm is structural: it lubricates a dry pastry exterior that has no sauce of its own and stops the roll reading as a third dry crust against the pie's two. The pie goes in hot so the warmth softens the pastry enough to bite cleanly through all three layers at once, because a cold pie in a cold barm is just dense on dense with nothing to bind it. Eaten fresh and warm from the bakery, it holds together as one thing rather than a roll with a pie skidding loose inside it.
The variations are mostly which pie goes in. Meat and potato is the standard; a steak pie makes it heavier; a Holland's-style flat-topped pie sits more evenly in the barm. The wider pie family, the pie sandwich, the pasty bap, the bridie, all share the same engineered-container reasoning from their own regions. Each deserves its own article rather than being crowded in here.