Across the East Midlands the roll is a cob, and in Nottingham and Derby it is the only acceptable word for it. The cob is the firm-crusted member of the bacon family: a round roll with a proper crust that crackles when you break it and a denser, less airy crumb underneath than a soft bap carries. It does not tear away in the hand; it cracks. Where a barm or a batch is built to be squashed flat, the cob holds its dome and gives the sandwich a bite of resistance before the soft interior. That crust is the defining variable here. The bacon inside is the constant every region shares; the firm-shelled cob is what makes this the East Midlands version.
The crust changes how the cob behaves under hot bacon, and it changes the build accordingly. Rashers fried until the fat has rendered and the edges have caught still shed grease and salt, but the crust holds its structure rather than going translucent, so the cob stays a roll you have to bite through cleanly rather than one that flattens into the filling. The crumb beneath takes a controlled amount of fat and goes dense and rich while the shell stays crisp, which gives a sharper textural step between outside and inside than the softer rolls manage. The build is the usual discipline at slightly higher stakes: butter while the cob is cool so the fat does not soak through, bacon in hot, sauce on the inside so it does not run down a crust that will not absorb it, eaten before the steam softens the shell from within. Done in time it gives a crackling top, a rich dense base, and bacon as the salt-and-chew line between.
Out of the East Midlands the same sandwich changes its word with the map. It is a bap across much of England, a barm in Lancashire, a batch around Liverpool and Coventry, a morning roll in Scotland, a stottie in the Northeast. Lose the bread question and it is a bacon butty, or a bacon sarnie when nobody is being careful about it. The fillings branch the way the names do: a fried egg with a yolk to be kept under control, a sausage with or instead of the bacon, the brown-against-red sauce question that each kitchen settles its own way. Those deserve their own articles rather than being crowded in here.