Kendal, a smallish market town in south Cumbria is celebrated for making this stuff, which, famously has been up Mount Everest because of its energy giving properties. It's white, square, and as hard as nails. Harder in fact: teeth-breakingly hard.
Quiggins are the people that originally made it, I think, and they started doing so, in Kendal, in 1880, although they had made confectionery since 1840, on the Isle of Man.
Anything this popular must have the de rigeur 'secret formula' and Kendal Mint Cake is no exception. Mind you, it's very, very minty indeed, so 'mint' is probably in there somewhere, I guess, in the form of peppermint essence. And the energy's got to come from somewhere, so 'sugar' is probably a fair bet too. From the taste of the stuff, I'd guess at sugar, sugar and more sugar being the other three ingredients. (Quiggins probably make toothpaste as a sideline, and have shares in private dental practices. Ho hum.)
Someone, somewhere in the history of Mint Cake, decided that it wasn't sweet enough (or, more likely, that revenue from the dental off-shoot could be higher) and covered the stuff in chocolate, producing the snappily titled 'Chocolate Covered Kendal Mint Cake'.
If you want to know what it tastes like, you can now order it over the internet (int technology marvellous?) - you could also put fifty Polos on your tongue (the sweet, not the car), and then drive at ninety miles per hour with your head out of the window and your mouth wide open. That'd work too. (And for the chocolate stuff, add some Cadbury's Bournville on top.)
Worth trying at least once. Have toothpaste handy, 'cos you'll want to scrub 'em for a week.