If you are swirling on some simple frosting with the back of your spoon for a true homespun look to your layer cake, then this technique will seem a bit trivial to you. But if you want to create a bakery-worthy, stunning dessert with smooth sides and pretty piping all around, then knowing how to apply a crumb coat will help you achieve your goal.
We wish there were a better term for the “crumb coat.” It sounds strange, but actually does describe what it is and does. Once your cake is filled, you might think you’re ready to frost your cake, and you are, but not for the final coat. First you apply a crumb coat (or base coat) of buttercream or frosting to the outside of the cake, completely covering the top and sides. This seals in any crumbs that might be on the surface and readies the cake for a smooth, final coat of buttercream. The crumb coat makes a huge difference in how the final coat goes on and, in turn, how professional it looks, setting the stage for the overall appearance of your final cake. Do not skimp on this step. It is easy, but it takes time because you have to chill your cake between applying the crumb coat and the final coat.
First, make sure that your buttercream is the right temperature. It should be very smooth and easy to spread. To apply the crumb coat, place your filled cake on a cardboard of the same size and then place on your turntable. Using an icing spatula (we like to use a straight edge at this point) place a small amount of buttercream on the top of the cake. Spread it around thinly; you want to create a smooth but thin layer of buttercream all over the top and then also on the sides. It does not have to look pretty; you will see the cake beneath the thin veil of buttercream. Just make sure to cover the tier completely, but lightly.
This is also a step where you should rectify any unevenness in the cake. Spin the tier around on your turntable and if one section dips down a bit, then add more buttercream to that area to build it up and create a level surface. Place the cake in the refrigerator and chill until the buttercream is very firm to the touch, about 2 hours.