I saw this over on The Negotiation Blog and it covered two of my pet topics, Negotiation and Willpower
The punchline is that your willpower/mental energy depletes glucose, so giving in to another temptation (like the cupcakes) helps avoid further depletion (and in the case of food-based indulgences, restores glucose). The whole willpower-as-a-muscle analogy seems to be getting a lot of support by recent experiments..
(added bonus: Dan Green has an excellent podcast (ConMan Talk) about various conflict management techniques. I recommend it to anyone who has to interact with other humans)