There are several stages of making the cake. Once each stage is complete, you’ll be able to get a reward from Gingerbread City.
When the cake making progress is complete, a new quest icon from an NPC should be available near the cake in Gingerbread City. In front of the cake near Bobo, talk to NPC Candy to initiate the quest and claim your Rudolph Mysterious Giftbox.
You may also watch this simple video walkthrough how to claim your rewards.