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!)
Can confirm. Per the (paywalled) Oxford English Dictionary [0]: 'barm' is The froth that forms on the top of fermenting malt liquors, which is used to leaven bread, and to cause fermentation in other liquors; yeast, leaven. OR The froth or foam on the surface of beer OR English regional (chiefly Lancashire). A soft round bread roll, a barm cake.
(So obviously those calling them cobs or rolls or whatever else are just wrong!)