Chill the frosted cake in the refrigerator for several hours, before decorating.
To make the black buttercream, it's helpful to start with chocolate buttercream. Add 1 tablespoon of the black cocoa powder, and 1/2 teaspoon of the black gel food coloring, then stir together well. Add more cocoa powder and food coloring until the buttercream reaches your desired shade of black. Note: The color will deepen and darken as the buttercream sits.Important Note: You should always sift the cocoa powder first, to remove lumps. Even tiny lumps of cocoa powder in your buttercream can clog a small piping tip, making it difficult to decorate your cake.