If you’ve ever marveled at the flawless frosting on a professionally made cake and wondered why your frosted cakes often end up specked with crumbs, here’s a tip: Apply a “crumb coat” to your cake before you spread on the actual frosting. If you use a little bit of frosting to seal the cake’s surface and secure loose crumbs, your final layer of frosting will go on much more smoothly.
Begin by brushing off any loose crumbs on the cake with a dry pastry brush or your fingers. (If you’re making a layer cake, the layers should already be filled and stacked.) Then spread a very thin layer of frosting over the entire surface of the cake. With lighter frostings, this layer may be transparent. Don’t worry: its role is simply to seal the surface. Fill in any cracks or divots in the cake.
After smoothing the surface of the crumb coat (don’t worry if it’s speckled with crumbs), refrigerate the cake for 5 to 10 minutes to let the frosting set. Before finishing the cake, clean the spatula, the rim of the mixing bowl, and the entire work area of any crumbs, which have an almost magnetic way of getting into the frosting. Now you’re ready to spread a smooth, crumb-free frosting onto the cake.
The simplest choice for a crumb coat is a bit of the actual frosting. Depending on the size of your cake and the amount of patching up you need to do, a crumb coat should take about 1/2 cup of frosting. I’ve found that I don’t need to make extra frosting, since I wind up needing less for the final layer.
As an alternative to frosting, strained preserves also make a fine crumb coat. The sleek, translucent coating that this creates is especially useful if you’re planning to finish the cake with a warm chocolate glaze. Melt 1/2 cup jelly, jam, or preserves with 1 Tbs. water until thin and smooth. Strain the warmed mixture into a small bowl and brush a thin layer onto the cake to seal the surface. Let it sit for 5 to 10 minutes to set up before applying the finish frosting. It should be somewhat tacky.