The butterie, the rowie of Aberdeen and the northeast of Scotland, is defined by laminated fat, and that is what separates the sandwich made on it from one made on an ordinary morning roll. The dough is folded and rolled with a heavy hand of fat the way a croissant is, but it is salty rather than sweet and squat rather than airy, baked to a flattened, irregular, deeply browned roll with a crisp, greasy, savoury crust and a chewy, fat-laden crumb. Split it and fill it and the roll is not a neutral carrier the way a soft bap is. It brings its own salt, its own richness, and its own crisp-then-chewy texture to every bite, so the filling is chosen to work with an assertive bread rather than to sit in a passive one.
The craft is restraint, because the roll has already done most of the work. A butterie is salty and fat enough on its own that the classic treatment is almost nothing: split warm, perhaps a scrape of butter that the laminated fat barely needs, perhaps jam or marmalade whose sweetness cuts the salt cleanly, or a savoury filling kept simple so it does not compound the richness already in the crumb. Bacon works because its salt and rendered fat run with the grain of the roll rather than against it; cheese works for the same reason. The roll wants eating the day it is baked, while the crust is still crisp and the fat has not gone heavy, which shapes it as a morning thing bought fresh rather than a sandwich built to be carried for hours.
The variations stay close to that salty laminated base and the breakfast logic around it. The plain buttered roll with jam or marmalade is the everyday reading; bacon in a warm rowie is the working-morning version; cheese, or cheese and a fried egg, pushes it toward a fuller breakfast in the same fat-rich shell. Each of those is its own balance against an already-salty roll and deserves its own article rather than being crowded in here.