The macaroni pie sandwich is carbohydrate inside carbohydrate, and the doubling is the whole idea. A macaroni pie is a Glasgow bakery staple: macaroni in a thick cheese sauce baked in a hot-water pastry case, a self-contained pie whose shell is already a starch built around a starch. Put that pie into a soft roll and you have stacked a third layer of bread around a filling that was two layers of carbohydrate to begin with. The defining fact is that this is a sandwich made of a finished baked good, not of loose ingredients: the roll is not holding a filling so much as holding another, smaller, sealed meal, and the appeal is frank, a hot, soft, cheese-and-pasta interior inside crisp pastry inside bread.
The craft is the roll and the temperature. A soft, plain morning roll is the usual carrier because the pie brings all the richness and structure and the bread's only job is to be a yielding shell around it that can be eaten one-handed; a roll with real chew would fight a filling that is already dense. The pie goes in hot, so the cheese sauce inside is loose and the pastry still crisp, and it is buttered, which on a build this rich is less a flavour decision than a way of bridging the bland roll to the salty cheese filling. Brown sauce is the common addition, a stripe of sharp acid run inside against a sandwich that is otherwise soft starch and fat from end to end and badly needs the cut. There is no other component, because the pie is already the meal and the roll is the way to carry it.
The variations stay on the Scottish bakery shelf. The pie eaten on its own, without the roll, is the un-doubled form; the Scotch pie in a roll runs the same logic with seasoned mutton instead of macaroni; the pie-and-beans and pie-and-chips builds push the carbohydrate stack further still. Each deserves its own article rather than being crowded in here.