Fondant is one of the cake decorator's best weapons -- a sort of edible sugar-based dough that covers cakes in an elegantly smooth and seamless fashion. Bakers are usually directed to freeze and thaw the cakes before covering them, and to first prepare them for the fondant by smoothing on a coat of buttercream icing. Although buttercream is the most traditional, other substances can be used to prepare the cake.
What the Buttercream Does
There are various reasons for covering the cake with buttercream before giving it its smooth fondant finish. The first, and most important, is that it acts as a sort of edible glue to help the fondant adhere to the cake. Another significant benefit for the cake decorator is that fondant is translucent, and the buttercream underneath prevents the cake from showing through the fondant. This is especially important when covering a dark cake. Third, the buttercream provides an additional source of flavour to compensate for the blandness of the fondant.
Substitutes for Adhesion
Although buttercream is the most widely used way to attach fondant to the cake, there are a variety of other ingredients that can be used instead. Marzipan is one option, providing a suitably sticky surface and a smooth, dough-like texture similar to that of the fondant itself. A variety of jellies or preserves can also be used, with strained apricot jam being a common choice. Strongly coloured jams and jellies will be visible through the fondant, and should only be used with fondant in compatible tints. Clear piping gel can also be used for this purpose.
Substitutes for Flavor
Although adhesion is the primary reason for a coat of buttercream, the added flavour is also welcome. Most of the substitutes previously listed will add flavour to a cake, with the exception of clear piping gel. Marzipan has an assertive almond flavour, and apricot or other jellies each bring a distinctive fruit flavour to a cake. Soft ganache, a mixture of cream and melted white or dark chocolate, will also enhance the flavour of a cake. A less-common alternative is commercial marshmallow fluff, which is agreeably sweet and sticky.
Substitutes for Visual Undercoating
Preventing the cake from being seen through the fondant is something not all substitutes are capable of. Clear piping gel or apricot jam should only be used when the cake underneath is pale enough to not show clearly through the fondant, and red or purple preserves should only be used with fondant of a compatible colour. White or dark chocolate ganache are suited for use under white or chocolate fondant respectively, and marshmallow fluff is an acceptable undercoating for white fondant.