The short answer is: I wouldn't worry about it--bake away and sell away, you'll be fine. Here's why.
Baked goods and, usually, recipes for baked goods are not protected by copyright law. Copyright only applies to original, creative "literary" works that have been recorded in some medium (TV, radio, online content, newspapers, etc.). So copyright law protects novels, magazine articles, movies, computer code, and Hubs! But not physical objects (say cupcakes) or even mostly recipes. What's the difference between, say, one of your Hubs (which is protected) and a recipe (which generally isn't)? It's kind of complicated and boring. (But see more here if it helps, http://www.copyright.gov/help/faq/faq-protect.html - there is question about recipes there).
Even if you say "Made by Betty Crocker" or even if you lie and pretend that it was your family's 500-year-old recipe from Sicily, it won't change the underlying fact that the recipe (or cupcake, say) just is not a proper subject for copyright protection.
Now, there are other laws that cover certain kinds of physical objects, such as patent law and trade secret law. There is also the law of trademarks, which protects brand names against copying or misuse. But, also for complicated reasons that I could get into if you want but would suggest not getting into because they aren't too interesting, none of these apply to your situation, I am pretty sure.
I also suspect that Betty Crocker would love nothing more that for you to provide them free advertising at a bake sale!
(If it makes you feel better, I am not pulling this out of thin air. I'm a lawyer. I don't specialize in this stuff, and of course this does not constitute legal advice, blah blah blah, and if you want real legal advice you should consult a lawyer, blah blah blah, but in the end, I would eat an entire box of Betty Crocker cake mix without any water if I was wrong about this).
Hope that helps, and good luck on the bake sale!