We love the Nutty Mutt bakery near us. They don't have cakes, but they do have adorable Birthday cookies! You might contact them and see if they can do a cake for you. My Sophia has a very sensitive tummy, so we have to be very careful, but we have never had any problems with treats from them!
Nutty Mutt Bakery
From their website:
"Nutty Mutt dog treats are freshly-baked, using only the best human-grade ingredients. All of the treats are corn and soy-free (many wheat-free), with no artificial preservatives, added salt or sugar, or by-products--ever."