ah i am shite at baking what you do, is get a plain cake from anywhere (asda do the plain white iced Victoria sponge thingamy in two sizes) go on ebay search for hannah montana cake and find people who will send you a real icing cake topper through the post. follow simple instructions to put on top of plain cake et voila.
look again at the ebay page; some are for fairy cakes and some are full size cake toppers. i just got one fo those and stuck it on an asda cake. i had ben 10 and dinosaur parties to do this yr. you can incorporate a pic of dd too if you canbebothered like; they email you a design based on what you ask for, for you to approve before printing it. and you can order way in advance coz they keep for months.