Wow, I haven't thought of these in decades. I tried one once and probably first heard about it from the late, great Colin Fletcher's book, The Complete Walker. Though, according this article from the San Francisco Chronicle, he gave them up and didn't mention them in later editions of his book. Still, I'm glad to see they're still available.
Here's the opening paragraph (from the article) as a teaser:
"In the early 1970s, a cloyingly sweet confection made in England's Lake District -- Wilson's Celebrated Original Kendal Mint Cake -- enjoyed a brief, and puzzling, spike in sales in the United States. Even more mystifying, the orders weren't coming from candy stores, but from shops that sold sleeping bags and hiking boots."