Most professional cake decorators rely on rolled fondant when their cakes must have an absolutely flawless appearance. Fondant is a special form of icing that you can roll into a flat sheet like pie dough, then unroll over the cake to form a perfectly satin-smooth surface. Like other cake-decorating skills, applying it takes practice, but it's within almost anyone's abilities. It's trickiest when the layers of your cake surround a soft cream or fruit filling, but putting the fondant on even these kinds of cakes requires only a small amount of extra care.
Preparing the Cake
Bake and cool your cake layers and prepare your filling. Trim the edges of the cake, if necessary, to make them completely straight and even. Place a medium-sized round or star tip in a pastry bag, and fill it with buttercream icing.
Place the bottom layer of your cake on a cake board. Place the tip of your pastry bag at one edge of the bottom layer, and squeeze the bag gently to pipe out icing. Make a ring all the way around the edge of the cake layer to act as a dam and keep the filling from squeezing out.
Fill the dammed area with your cream, fruit or fruit-and-cream filling. Place the second layer of cake on top, pressing down just firmly enough for it to make a good seal on the icing dam.
Thin your buttercream slightly with milk, so it will form a slight crust as it dries. Spread buttercream thinly but evenly over the top and sides of the cake. Carefully smooth any slight bulges where the buttercream dam holds the two layers together with your cake-decorating spatula. If you see a slight indentation, carefully fill it with the buttercream until it's completely smooth. The smoother your icing layer, the smoother your fondant will be.
Rest your cake for at least an hour, until this "crumb coat" of icing has hardened. For cakes with cream-based or other perishable fillings, keep them refrigerated during this time.
Covering With Fondant
Weigh out the amount of fondant you'll need to cover your cake. Cake decorating books and websites usually contain this information, or it may be printed on the packaging for your fondant.
Clean your counter scrupulously, and wash your hands. Knead the fondant until it's pliable, then dust the countertop lightly with powdered sugar.
Roll the fondant into a circle or rectangle large enough to cover your cake. Add the width of the cake to the height of its sides plus a little extra for convenience. For example, for an 8-inch round that's 4 inches high, you need a circle 16 inches in diameter plus another 1/2 inch or so. For a 2-inch high, 9-by-13-inch sheet cake, the sides add 4 inches to the fondant's area, so you need a 13-by-17 inch rectangle of fondant, plus an extra 1/2 inch on each side.
Position your rolling pin at one edge of the fondant sheet and roll the fondant around it. Lift the rolling pin, with the fondant wrapped around it, to the cake and unwind the fondant so it drapes over the top of the cake and down the sides.
Smooth the fondant over the top of the cake using a cake smoother or your own clean and well-dried palm. Don't press any harder than necessary, or you may squeeze out the soft filling. Next, smooth the sides into place, trimming away any excess where the fondant folds over itself. Smooth the trimmed areas until they're not visible. Finally, trim away any excess fondant at the bottom edges of the cake with a sharp knife.
Finish your cake by decorating it with buttercream or other accents, especially at the bottom where the cut edge of the fondant would otherwise be visible.