My dream cake would be Celestia's favourite cake. Now if only they could air that sometime in the show that'd be perfect. We must follow Celestia's cakely rules!
I've had one of those, for my birthday. Mine also came with some 100s&1000s on the top. (Sorry to burst your bubble, but it didn't taste particularly good. The rainbow bit inside just didn't have any flavour and the icing was painfully sweet.)