I hesitate to say this because people get the wrong impression and "expect" Disney to do this ....... we were talking about DH's birthday on check-in and were sent a complimentary chocolate cake that was rich and decadent. There were only 2 of us, and we love chocolate, but we each had 3 servings from this tiny (~ 6") cake because it was so rich. Don't know if this is one of the choices you can purchase or not, but if you love chocolate, this is great. Just don't be surprised if it looks very small.
You can buy Haagen-Daz ice cream to go with it in the Mercantile store.
I'd still mention it when you make your reservation and mention it at check in, but just don't count on anything. There are probably thousands of people celebrating something on any given dayat WDW.
In case you don't know about the Birthday Button, you can pick one up at the MK (and I believe other parks, too). Ask when you go in the park and they'll direct you. CMs will notice it and give Happy Birthday wishes throughout the day. Hope you have a great trip!