The butterie is what makes this an Aberdeen sandwich, and it is unlike every other roll in the bacon family. Known locally as a rowie, the butterie is a flattened, irregularly shaped roll laminated with a heavy ratio of fat folded through the dough, then baked until the layers crisp at the edges and stay dense and chewy in the middle. It is salty in its own right, faintly greasy to the fingers, and closer in build to a savoury laminated pastry than to a soft white roll. Where a bap or a barm is plush and absorbent, the butterie is rich and resistant, already carrying much of the fat that other regions get only from the bacon. The bacon is the constant across every region; the butterie is the part that makes this version distinctly Aberdeen, and the most decadent in the family.
Putting bacon into a butterie is an exercise in fat on fat, and the roll is built to take it rather than be overwhelmed. Rashers fried until the fat has rendered and the edges crisped add salt and a brittle crunch to a roll that is already short and flaky, so the texture runs to crisp-on-crisp with a chewy, buttery core holding the centre. The build is restrained because the butterie does not need help: it is often warmed so the laminated layers loosen and shatter cleanly, the bacon goes in hot, and butter is usually redundant since the roll supplies its own. Sauce, if it appears at all, goes inside and sparingly, because the butterie is salty enough that brown or red is a smaller decision here than it is anywhere else. The reward is a roll that crackles at the edge, gives a dense buttery chew in the middle, and carries the bacon as a salt-and-crunch line through the centre.
Travel out of the northeast of Scotland and the everyday version of this is built on a softer roll under a different word. It is a morning roll elsewhere in Scotland, a bap across much of England, a barm in Lancashire, a cob through the East Midlands, a stottie around Newcastle. Strip the bread question away and it is a bacon butty, or a bacon sarnie in passing. The fillings stretch the way the names do: a fried egg whose yolk has to be managed against all that fat, a sausage with or instead of the bacon, the brown-against-red sauce question that divides households. Those deserve their own articles rather than being crowded in here.