I, too, think you've handled it well, Sher, and that you should now ignore her 12-year-old behavior. Got that list of supplies written up yet?
When I was new at my previous job, a co-worker told me that the way they did birthdays was, I should give her a dollar and then when my birthday came around she'd buy a card and get everybody to sign it. I told her the card wouldn't mean much if I had to pay for it myself, and that I would hope I could make friends who would WANT to celebrate my birthday. (and I did).
It just seems to me that this is not really about this woman's birthday, it's about some issue she has with you. Dunno why. I'd say don't feed it. But do let us know what happens today! And if she's gonna ignore those chocolates, take 'em back! It's a sin to waste chocolate.