I found someone writing
Der einmaligste Kuchen, den du jemals gesehen hast.
For me as a German this sounds strange. Is there really a comparative or even a superlative for einmalig? I mean, either something is unique or something is not. Imho, there isn't anything "uniquer" than any other thing.
(ah and maybe someone with enough reps wants to create some tags for comparative and superlative?)
The form is constructed according to the normal way of forming superlatives, and you immediately recognized it as such. I'd say this means that the intent of the speaker came across to you load and clear, so we have no choice but to accept that someone formed a superlative for "einmalig" and you understood it.
That doesn't mean that it's in any way useful, "reasonable" or "correct" to do this - there could be any number of prescriptions "prohibiting" it. In this case, obviously it's unnecessary to form superlatives for a property that is incomparable by definition. But just as anti-theft laws don't actually prohibit theft, anti-nonsense rules don't actually prevent nonsense from being said. The question whether this sentence "really" contains a superlative or not is therefore down to definitions.
It isn't a superlativ, it is a Hyperlativ.
While people knowing German grammar will balk at its usage, it is used, mostly for
a hypothetical thing which is impossible because it combines advantages without disadvantages: "Das optimalste Farmtier wäre die eierlegende Wollmilchsau."
a thing which is so fantastically good that it dwarfs other things, so you adjust the language to show your enthusiasm: "Das ist der einmaligste Kuchen, den ich je gegessen habe" (Die bisherigen Kuchen waren kein Vergleich).
