Lembas has been a running debate in the Hobbit Hole since nearly the beginning. I insist they are digestive biscuits (which aren't biscuits in the American sense, but in the British "cookie" sense but trying to get that through to anyone is a lesson in futility!)
;-D
I hypothesize that Tolkien needed sustenance for his characters and he happened to have a plate of digestive biscuits with his tea and it likely sustained him while he was writing and so that is what he used.
Well, with that grand funkle, I suppose I should try going back to bed.
"plate of digestive biscuits"
I missed that on the breakfast menu this morning.