Your health is more important than her ego, RT. If you can't come right out and refuse just tell her you like to take it home and have it for dessert. Then work up to just saying no. But you know....there may be one or two other people in the office that would also love to turn down the cupcakes and are looking for someone else to do it first.
Welcome to the cave. Such a great first post Your avatar is just fine! I've been lazy but will upload a pic of myself eventually too.
"Flab grenade" and "death cake"....two phrases I wish I had chosen for my username....haha.