I am not sure how much that counts as willpower. Willpower, often, has to do with the ability to revert preference reversal caused by hyperbolic discounting. When both the rewards are far away, we use a more abstract, rational, far-mode or system 2 reasoning. You have rationally evaluated both options (eating vs. not eating the cake) and decided not to eat. Also, I would suspect that if you merely decide this one day before and do nothing about it, you will eat the cake with more or less the same probability if you haven't decided. However, if you decide not to eat but take measures to not eat the cake, for instance, telling your co-worker you will not eat it, then it might be more effective and count as willpower.