butterbeer is supposed to taste similar to cream soda... there are sites you can purchase cream soda concentrate much like you can root beer. if you google butterbeer, you can find recipes for it and pull the spices into the cake. but I'd find a rootbeer cake recipe you have tried and use the cream soda concentrate.
universal serves their cold butterbeer with a butterscotch foam, that could be an option for icing flavor. and i found this recipe for butterbeer http://www.foxnews.com/leisure/2010/07/02/harry-potters-butterbeer-recipe-uncovered/
sad to say, my harry potter knowledge seems to be disappearing from my head lately lol.
its probably gonna be some trial and error. I've gone thru 3 root beer cake recipes and still havent found the perfect one yet. but experiment with the root beer concentrate as its soooo much cheaper lol
good luck and if you find one that works... i would love it if you would share... my sis n law was my harry potter buddy and her bdays coming up.