A crumb coat is a term that comes up fairly often for those of us who like to bake cakes, but it is not often well defined in recipes. Sometimes, it isn’t mentioned at all, although anyone who has ever tried to frost a cake and ended up with lots of crumbs stuck in the frosting would certainly find that knowing how to apply a crumb coat is handy skill to have.
A crumb coat is name of a very thin layer of frosting that is applied to a cake before the final frosting is put on. This coating essentially traps all the loose crumbs on the surface of a cake in a thin, sticky layer of frosting. With the crumbs secured, your final layer of frosting will glide on smoothly and cleanly, without picking up a single crumb from the cake itself. The crumb coat is made from the same frosting that will go on the rest of the cake, so there is no need for a special batch of frosting or an additional recipe. There is no special technique in applying it: just use an offset spatula or knife to apply the frosting and smoothing it down until you can see the cake underneath. I like to allow the crumb coat to set up for a few minutes (although it doesn’t make that much of a difference in the end), then apply the rest of the frosting, pulling it lightly across the base layer. The coat will work with any spread-on frosting (pour on glazes and ganaches don’t generally need a crumb coat) and dedicating a small amount of your frosting to it will usually make a big difference in the presentation of the finished product.