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

I can’t not believe I can’t believe it’s not butter, but when I do, I believe it.

And then there’s English’s double negative which results in single negative somehow

“I ain’t doing nothing” means “I am not doing anything”

but i’m not always so sure…they should call it I Could Believe It Might Be Butter