That’s not what this double negation cancels to, though. The cancellation of “I can’t believe it’s not butter” is “I must/I’m compelled to believe it is butter”. This is because you’re saying you lack the power to believe that it is ~B, where B is the set of butter. Thus, because you’re addressing that it exists (i.e. you have to believe it’s something), you not only believe it is in set B but are powerless to do otherwise. ∎

See also: /r/isitbutter