I wouldn't stress about it but keep a close eye on her in case she does react to the cake she had eaten. Maybe look up the after hours vet phone number and keep that on hand if you don't feel right.
If it helps, one day I left a bowl of M&Ms on the coffee table when DH and I went out for the day. When we came back, I went to grab some M&Ms and the bowl was empty! I actually scolded DH for eating them and he looked at me in puzzlement and said that he didn't finish off the bowl. We eventually worked out that it was our puppy that had jumped up and ate the WHOLE bowl of chocolate M&Ms! For some reason, I didn't think my puppy would be able to jump onto the coffee table as we have another older dog who doesn't jump on furniture.
After panicking a little, we figured out that since it would have been a few hours after she had eaten them, it would be ok and she was fine - no side effects thank goodness. Its just what dogs do!
Your dog sounds adorable! She probably thought all her christmases had come at once