The marketing genius of this song is that not only are the first three bars totally hum-able, but that the last bar lands the product name in a way matched neither before nor since.
Think about it: "Gimme a break, gimme a break, break me off a piece of that Kit Kat bar." What else could possibly replace the last three words? Nothing, that's what. And the idea of breaking off a piece of a Kit Kat is so perfectly right on for the four sticks that make up the bar that it's bound to appeal to your sweet tooth. It's genius, just genius. You know, in an evil, mind-eating kind of way.