Employee brings homemade cupcakes to the office, coworker sends a passive aggressive message when she doesn't get one

Are you entitled to the food that your coworker brings in? 

This is a pretty easy "no." Just because someone brings in food doesn't mean that you're entitled to it. If they brought enough to share- a rare act of kindness that should be celebrated- then you're lucky to get one. You don't automatically earn one just because you exist and you showed up to work that day. This kind of entitlement is difficult to swallow when you come across it in another person, because what are you supposed to do about it? You can't let them just run around thinking they're entitled to everything, but you also can't swoop in and demand that someone give you everything you want. 

If someone is nice enough to bring baked goods to the office, then you should spend your time thanking them. You shouldn't get mad just because they ran out before you could get there. This is no way to conduct oneself in the office, especially if you don't know the other person that well. What gives you the right to barge in and demand that they do everything your way? Where did that sense of entitlement come from? You better be sure that it doesn't rub off on anyone else, lest you find yourself in a conflict that you can't get out of. It's better to keep things on an even keel at work when you can, and when you can't- just to let all of that go.