Prevent Excess Crumbs

• Crumbs are unavoidable when you frost a cake. But you can take steps to minimize the amount of crumbs that develop so that it is easier to apply an effective crumb coat. Cake is more likely to crumb if it is still warm. Allow the cake to cool to room temperature before removing it from the pan, assembling it or frosting it. Spreading frosting across the cake generates friction that breaks crumbs free. A cool cake is less likely to yield crumbs than a warm cake.

Prepare the Buttercream

• Spoon about one-fourth of your buttercream into a separate bowl. Add a small amount of warm water to thin the buttercream slightly. Thinned frosting should be soft and still opaque. Do not thin it into a semi-translucent glaze. It should remain firm enough to stay where you apply it; it should not run. Regular buttercream tends to be thick and stiff, and it is more likely to cause excess crumbling if you apply it to bare cake. Thinned buttercream is easier to spread. While glaze would be the easiest topping to apply and might be successful at trapping all the crumbs, it would be difficult to apply the top coat of buttercream on top of a glaze.

Apply the Crumb Coat

• Remove the cooled cakes from their pans and apply regular, unthinned buttercream between the layers. Spread a dollop of thinned buttercream on the surface of the cake with a frosting spatula or butter knife. Spread the frosting in long smooth strokes. Expect crumbs to be visible in this layer of frosting. Dip the spatula in warm water then wipe it clean with a paper towel to eliminate excess frosting. Buildup on the blade can pull crumbs loose on the surface of the cake. A warm blade spreads frosting more evenly. Continue applying thinned buttercream across the entire surface of the cake. The crumb coat should be relatively thin -- patches of cake may be visible through the frosting. Let the crumb coat dry before you apply the top coat.

Apply a Buttercream Top Coat

• Spread the regular buttercream on the cake in short strokes. Continue rinsing and wiping the blade between strokes. The top coat should contain well over half the buttercream that you started with. By applying a lot of buttercream in short strokes, the sheer volume of the second layer should trap any remaining loose crumbs. Once you have covered the entire cake with a second layer of frosting, you can use longer strokes to even the surface of the cake.

Choose an Alternate Topping

• Buttercream crumb coats are good primers for many cake toppings. It’s easier to apply any thick, spreadable topping such as ice cream, whipped cream or a frosting that contains mix-ins if you apply a crumb coat beforehand. Rolled fondant adheres to frosting more effectively than it sticks to bare cake. Poured toppings such as caramel or ganache also spread evenly across a crumb coat as long as the topping is not too warm, which could melt the buttercream and cause it to run.