Just a question for the OP, has she ever refused to share other recipes?
No, I have always been quite happy to share.
And have very gracefully taken no as an answer several times before. Though the person saying no is usually kind enough to offer a reason. Mostly the reasons are family secret. One was because she didn't want it ever to find its way back to her MIL and the only way to be sure was to never give it out.
I don't think you understood the question Marcel was asking. 
Sorry. Not that I know. I have not asked her for any other recipes and do not know if others have asked her for recipes and been outright refused.
Is she generous in other ways? Do you like her as a friend for reasons other than her cupcakes?
Actually I think it's pertinent if OP's friend accepted recipes and gave others in return vs Op's friend accepted recipes and has never to Op knowledge given one out. It's not abnormal to be annoyed to find out your friend has a "neither a borrower or lender policy " but is willing to bend on the borrower bit. I also understand " the best cupcakes ever" might be the recipe equivalent of asking to borrowing someone's entire lifesaving.
Feeling a friend does not reciprocate is pretty damaging. ( I'm responding to multiply posts about OP's feeling not just the above quoted )
I also don't think everyone thinks of any "best ever " recipe as something so beyond sharing It's rude to ask.
Was friend rude to not even acknowledge the request I think so. I personally hate coy "no" but even the coyest response would have been better then nothing.
dotty you made the point that people can pick up on feeling , and that possible OP's friend pick up on OP's feeling she was a little bit entitled to this recipe. Normally I agree but I do think E-mail can hide those nuances much better then face to face conversation , so
I think it's possible that OP asked is such a way that her friend did not pick up on how OP would feel.