The Giant Cake is a final challenge boss in the Birthday Realm (not the similar Cake Realm).
The Giant Cake was specially baked for Bob's Special Day, but the cake didn't like it and went wild against its would-be consumer, I wonder if it is somehow related to secret ingredient. Who wondered that it will have whole Portal inside it.
After defeating the boss, the player receives the 
Golden Cherry as a reward, and access to the Cake Realm.