The thickness of buttercream frosting helps to conceal unevenness in cakes, which tend to rise more in the middle than at the edges. The frosting’s consistency also makes it difficult to spread evenly, however. A few tricks make it easier to spread buttercream frosting smoothly and produce an even finish on your cake.
Preparation and a Specialty Tool
Use room temperature frosting, which is softer and smoothes more effectively than chilled frosting. If you need to raise its temperature quickly, fill a bowl with warm water and place the container of frosting in the bowl. Stir the frosting to evenly expose it to the warmth. Frost the cake with an icing spatula, which looks like a very long butter knife. An icing spatula enables you to make broad strokes, yielding a more consistent surface.
Create Layers of Frosting
Apply two layers of frosting to your cake. Make the first layer -- the “crumb layer” -- relatively thin, about 1/8 to 1/4 inch thick. The crumb layer traps the loose cake crumbs that would otherwise make the frosting look bumpy if they appeared too close to the surface of the finished cake. Make the second layer of frosting thicker, about 1/4 to 3/4 inch thick, depending on how much frosting you want on the cake. Use the outer layer of frosting to smooth the appearance of the cake overall.
Application Trick
After you spread each dollop of frosting with the icing spatula, dip the spatula in a cup of warm water and wipe it with a paper towel to remove the water and wet frosting. This keeps the spatula warm, which warms the frosting and creates a smoother finish.