Items you will need
Dish soap and sponge
Labor-saving impression mats prevent you from piping dots, diamond or other textured patterns into an icing cake. Most mats can be used with either fondant or with crusting buttercream, which crusts over on the outside. Work in a cool room so the frosting won't heat up and soften. The key to success in imprinting your cake is a good eye, a steady hand and attention to detail.
Frost your cake with crusting buttercream, using an offset spatula. Create an even 1/2-inch layer on all sides of the cake. Allow the cake to sit at room temperature until the buttercream has crusted over, about 15 minutes. If you try to impress the cake before the buttercream crusts, you'll end up with a messy pattern because the buttercream is too loose or wet to hold a design. Once it crusts, it's firm enough to do this.
Coat the textured side of the impression mat with a thin coating of cornstarch, which will prevent the mat from sticking to the frosting. Sprinkle the cornstarch by hand then shake the mat to spread.
Press the mat into the side of the cake, starting 1 inch in from the right side mat edge, so you don't imprint the edge of the mat. Press gently but firmly to impress the design onto the buttercream.
Pull the mat away. It should come loose easily.
Match the impression mat to the edge of the pattern on the buttercream -- where you left off -- then press the mat into the cake again to continue with the design. Work this way all the way around the cake. Then press the mat into the top surface of the cake, if desired.
Wash your impression mat in warm soapy water to clean it. Store until the next use.
Test your buttercream to see if it has crusted by pressing a finger to it lightly. If it has crusted, you'll feel resistance when you touch it, and when you pull your finger away, it won't have frosting on it. If it hasn't crusted yet, the frosting will stick to your finger.
You can use impression mats in two ways -- to either imprint or emboss the cake. Pressing the protruding, patterned side of the mat into the cake imprints the frosting; leaving the protruding side sticking out toward you embosses your cake.