Mike will likely give the best answer on this, but until he does...
I have a few tins but also had a bulk jar that is gone . Towards the end of the jar, I hadn't touched it for months and it was quite dry... but the aroma was striking when I opened the jar... cinnamon... I couldn't believe it... but it was wonderful smoke dry.
Now await for Mike to jump in to give you the answer to your question.
Cinnamon, maybe. Chocolate, probably. Maybe some sort of chocolate liqueur. Something about it reminds me of a Far East food flavoring. A spice of some sort that I cannot place. I just had my nose in a tin and I cannot quite figure it out.
Sorry to disappoint but I never did fathom the topping. I believe however that it was complex, comprising at least 3 agents, maybe more. Over the years, as related to me by smokingpipes.com, Bracken Flake has had various incarnations. I know of three but it is difficult to say which reigned just when.
I just traded for 2 tins that were supposed to be from 3/07 but I think were mislabeled as the contents, although a good, smoky, medium-bodied smoke, were what I understand to be current production.
My chronology would be:
2-back '02, '03-6/07
1-back 6/07 - fall '08
current - fall '08 - ?
OK, so as not to disappoint, I've had my nose in a jar of what little I have left of the 2-back version, and I would say it has light tonquin, light floral, chocolate topping. Other than the tonquin, the rest is all a shot in the dark.
Oh, and while on the topic, if anyone has any of the two-back version that they could part with, you would find me eager to trade.