For many years, I have noted a quality in some (not all) very good bourbon or rye which adds complexity and interest. I have found it hard to describe this taste, at one time I used the term here "old curtains" which aroused some amusement. When you think of it, it's not such an odd expression since good textile is (I believe) from vegetable matter such as cotton (just an example) and of course grains are a growth of the earth too.
Recently, I was reading this review of Maker's Mark CS:
And there is the term, marzipan. And I can see exactly what he meant by that term. Just the other day an Eagle Rare (current one, 10 years old) had exactly that character.
And I've actually detected it sometimes in MM, but not always and clearly the barrels selected for the CS bottling had it and the reviewer is right the higher proof (112) would showcase it more. I believe the taste may actually be some kind of ester that is produced by the aging process, by the oxidative breakdown of some of the alcohols in the whiskey but I'm not 100% sure.