A crumb coat, also referred to as the “dirty icing“, is the base coat of icing on a cake. It is done to seal the cake and prevent any stray cake crumbs from getting into the final icing coat. Once the crumb coat is in place, the second layer of icing will go on cleanly and the finished cake will look smooth and uniform instead of rough or “dirty”.
To crumb coat a cake, first level the cake with a sharp knife. An even, level surface is important when stacking multiple layers. On a cake plate put a dollop of icing on the bottom and then place the first layer of cake on top of it, this will prevent the cake from shifting around. Fill each layer and stack the cakes one at a time. When adding the last layer, flip the cake upside down so that the bottom is the top. This will reduce the amount of crumbs that get into the crumb coat. Apply a thin layer of icing around the sides and top of the cake then smooth it out with a bench scraper or offset spatula. Chill the cake for about 20 minutes to set the layers and the crumb coat. Finish the cake by applying the second layer of icing and decorations.