A crumb-coated cake is one that is sealed with a thin layer of frosting before decorating with the final layer of frosting. The purpose of the crumb coat is to seal in loose cake crumbs, which makes it a simple task to frost the cake without the risk of stray crumbs marring the surface. A crumb coat is particularly useful when applying light-colored frosting to a darker cake.
Making a Crumb Coat
Make a crumb coat from the same frosting you will use to decorate the cake. Reserve a portion of frosting for this purpose. After applying the crumb coat, discard any remaining frosting from this portion to avoid introducing crumbs to the exterior frosting.
Applying a Crumb Coat
Spread a layer of frosting on the cake, thinning it down until you can see the cake through the frosting. Fill in small imperfections in the cake's surface at the same time to create a smooth, crumb-free surface.
Some cooks put the cake aside for a few minutes or refrigerate it for a half-hour to let the crumb coat set before applying the final frosting.