To all that come to his happy place: welcome — Walt Disney
My husband has a summer birthday, and it just so happens we’ve been at Disney TWICE during his birthday. Disney does a great job of helping people celebrate all kinds of celebrations…weddings, anniversaries, adoption, family reunions, and especially birthdays. In fact, we had the most wonderful cake at our on-site wedding. It was a beautiful, two-tier chocolate cake, with white chocolate raspberry filling, and buttercream icing. The staff decorated the cake with little Mickey heads and of course, a Mickey & Minnie cake topper. This was part of the wedding package, so costs are different, but any cake from Disney is pretty yummy.

A few years ago, we rented a specialty cruise (pontoon boat) from the Contemporary Resort and ordered a special Mickey birthday cake. It was a beautiful chocolate ganache, which is absolutely delicious. The pontoon driver took us around Bay Lake, then found a great fireworks spot to enjoy Wishes, with the music piped into the boat. It was a very memorable (and higher priced) experience, which requires at least 90 days advance reservation (407-wdw-play).
On our most recent trip, my husband and I decided to have a special dinner for two at Kona Cafe in the Polynesian resort. Most restaurants will have a basic 6-inch cake available, however it seemed like a lot of dessert for just the two of us. Instead, our waiter was able to bring him a special birthday cupcake plate, which was just right. There is no guarantee that a cake will be available when you walk up, but it doesn’t hurt to ask.

If you want to make sure a cake is available or to something more than a basic chocolate or vanilla cake, call the Cake Ordering Hotline at (407) 827-2253. Since the bakery at the Grand Floridian makes the cakes for the on-site weddings, they have the most options as far as flavors. I highly recommend calling the main line for ordering a cake 2-3 weeks in advance to get the most options.
If you are looking for more information on ordering cakes, AJ from Disney Food Blog has a great article.
Have fun exploring the World!