Seriously? I think I just came in my pants…
Musk sticks? Most people outside of Australia don’t even know Musk is a flavor, let alone try to imitate it …
They’re not unique but apparently Reese’s has some secret for making peanut butter cups that taste good. I once made the mistake of buying a Russell Stover peanut butter cup. It was so awful I threw half of it out.
That’s the candy company for dieters. It’s a treat that’s also a punishment. 
There’s at least one other brand with the same flavour and texture - they’re not cube shaped (they look like peanut butter cups, with foil wrappers), but otherwise they’re the same.
Does anybody besides Canfield’s make a diet chocolate soda? Are they even in business any more?
Nut Goodie bars - milk chocolate, peanuts and a maple filled center. They were one of my favorites as a kid.
Hard to find (I don’t think I’ve ever seen them in a store in KC) - every now and again my mom finds them and sends me a box.
I can’t pinpoint the source but I once read an article saying that was intentional on the part of Hershey’s. Supposedly when the company was developing their milk chocolate bar over 100 years ago, there was the belief that American consumers wouldn’t buy a chocolate bar that was too chocolately. That’s why the Hershey Bar has a cheesy taste to it. (That’s also why I think Hershey’s dark chocolate Special Bars taste a lot better.)
As for unique candies, what about Tootsie Pops? Or the unfortunately-named Charm’s Blow Pop?
Chick-o-Stick is pretty much the same thing as Zagnut.
Zotz. (There is a mention of Zotosies that are similar, but no evidence they actually exist outside of a Wikipedia citation.)
You’d have to find something that’s either extremely hard to make, or very unsuccessful, or restricted to a tiny regional or ethnic market. Or the recipe is complicated enough that it can be successfully protected by intellectual property laws. The problem is that snack food is generally a low-margin, commodity product, which means that it has to sell in huge quantities and be cheap to produce. Which is why snack foods are basically very similar, and all the effort goes into the marketing.
Really, unique snack foods are a bit like rare elements of the periodic table; they exist, but their half-lives are so short that they disappeared from the natural environment billions of years ago, and nowadays they only exist in the lab or very briefly in the heart of stars. Very briefly, in the heart of stars. That’s where love comes from, don’t you know? It’s generated in the heart of stars. It exists very briefly. And it powers the world.
I was going to mention Peperami - the meaty pork snack that’s 44% pure fat - but apparently it’s not just sold in the UK, it’s popular in Germany as well, and according to Wikipedia’s article there are similar products (mainly sold in Germany, where they love pigs and also solidly-built women).
In my opinion Very Briefly, in the Heart of Stars would be a great name for an album by a female singer-songwriter with a pixie girl voice e.g. Joanna Newsom. Or it would be a good signature block.
I (almost) hesitate to ask about your basis for comparison. 
Nerds?
Try here, the same company that Jeff Lichtman linked above.
I was comparing them to Necco Wafers.
Crunchy Frog
Spring Surprise
Necco Wafers are, at least to my taste buds, incredibly tasteless and chalky. Smarties taste more like powdery SweeTarts, though less strong.
Apparently they also made Bit-O-Chocolate and Bit o’ Licorice bars, but I don’t remember ever seeing those…