They Deserved the Ridicule
Rick-Rolling someone is one of the most archaic memes in the history of memes. That does not detract from the sheer power behind those lyrics. If your boss cannot write on a third-grade level, you have the utmost authority to belittle them in your resignation notice. That, and it's illegal to withhold payment.
Baking Your Way Out the Door
Evidently, Mel's boss got angry about her "wasting materials" to make herself and her department this resignation pastry. But, the members of her department loved it. So, why complain when you have a cake to eat? What are you going to do? Fire her?