It’s barm, with an m, from barm cakes. Barm is the frothy scummy stuff you get on top of fermented beer, and it contains yeast. Back in the day bakers would take the barm from local brewers to make weakly fermented bread rolls… barm cakes.
(So obviously those calling them cobs or rolls or whatever else are just wrong!)
(So obviously those calling them cobs or rolls or whatever else are just wrong!)