One hallmark of a professionally iced cake is immaculate frosting. A homemade cake, on the other hand, can look less than perfect because during icing, fragments and crumbs find their way into the frosting, leaving it lumpy or freckled. Home bakers can achieve the professional crumb-free results if they follow the same tips and tools that bakers use when decorating their cakes. Using chilled cakes and applying a crumb coat keeps the cake's final appearance smooth and free of crumbling.
Prepare the Cake
Turn the cake out of its baking pan and cool it completely before moving it to the refrigerator for four to six hours. A chilled cake holds together better than a room-temperature or warm one, and fewer crumbs will lift from the cake's surface during frosting.
Remove the chilled cake from the refrigerator and gently sweep loose crumbs from every surface that will meet a layer of frosting. Use clean hands and a light touch; the purpose is to remove any barely attached crumbs, not to stir up new ones. Save the crumbs that the cake sheds as a topping, or discard.
Add 1 to 2 tbsp. of frosting to the center of the serving plate, then move the cake to the cake plate. Press the cake lightly to affix it to the plate. This dollop of icing acts as a glue to hold the cake in place during frosting.
Spread the filling frosting atop the cake's bottom layer - if the cake is to have multiple layers. Add successive layers in the same fashion, frosting the top of each layer before placing the next. Skip this step if the finished cake has one layer.
Apply the Crumb Coat
Set the cake plate bearing the assembled, but unfrosted cake on the turntable or lazy susan. A turntable lets the cake move under the spatula instead of forcing the decorator to move the spatula over the cake and risk tearing away pieces of cake.
Mix 1 to 2 tbsp. of water with sufficient frosting to make a thin, but still workable paste slightly thicker than pancake batter. This thinned frosting is the basis of the crumb coat, an inner layer of frosting that traps crumbs within it so that they don't mar the cake's appearance. The amount of water and frosting the crumb coat needs depends on the size of the cake, the ambient temperature and the humidity.
Apply the watered frosting to the cake in a thin layer. Keep the application even and light. If crumbs appear in the frosting, leave them; they'll remain trapped in the crumb coating where they cannot migrate to the outer layer of smooth frosting.
Refrigerate the cake for another hour or two if possible to let the crumb coat set. The cake's crumb coat should become firm to the touch, giving the outer layer of frosting a solid foundation on which to glide.
Frosting the Cake
Apply a generous portion of frosting to the top of the crumb-coated cake once its surface sets. Move the cake under the spatula rather than maneuvering the spatula across the cake - this will minimize stress on the cake's interior.
Frost the sides of the cake with additional icing, using more than the cake seems to need. Think of frosting as the snow beneath a skier's skis; the spatula is the ski, and it can only glide where there is sufficient snow to keep it moving freely.
Remove excess frosting from the sides of the cake to leave the surface smooth. Hold the clean spatula parallel to the cake's side and spin the turntable slowly to move the cake beneath the spatula. Wipe off excess frosting as it collects on the spatula's surface.