We had the most delicious cake from the Boardwalk Bakery on our last trip (check TR for pics!) We had to order a 10" cake to be able to choose the flavors.
Just so you know the difference, the Cake Hotline is for when you want a cake to be served at a restaurant meal (and if you want the Grand Floridian Bakery making your cake, you'll need to be eating at one of the GF restaurants).
If you want a cake delivered to your room, call Private Dining at the resort where you'll be staying.