A fondant cake is not just the outer layer. The cake itself is first baked and then covered in a "crumb coat" of buttercream or ganache. This layer serves two purposes: it seals in the cake crumbs to create a smooth surface and acts as an adhesive to hold the fondant in place. The fondant is then rolled out into a thin sheet and carefully draped over the entire cake, and the baker smooths it out and trims the excess.
