Cake decorating has come a long way from the days of piped-on "Happy Birthdays" and icing rosettes. It's now a breeze to personalize with images or transfer pictures onto cakes. For detailed, lifelike images, you can have edible icing sheets printed that can be affixed to fondant or buttercream; for simple designs, such as cartoon characters, employ a "fill-in" method using buttercream. Use transfer sheets if you don't have much time or you have an intricate design, but if you have a few hours to spare, try your hand at filling in the design.
Edible Icing Sheets
Peel the plastic backing from the edible icing sheet and cut the design you want to apply to the cake using scissors. To cut straight edges uniformly, use a straight edge and razor blade or a guillotine paper cutter.
Apply a thin layer of gum-paste glue to the fondant using a pastry brush. If you have a buttercream icing, you don't need gum-paste glue.
Position the transfer face down on the cake. Lightly press the transfer to affix.
Buttercream Filling
Print out your selected picture in the size you want it to appear on the cake. Trace the image on a sheet of parchment paper using a fine tip marker. Mark the outer boundary of the cake as well.
Turn the parchment paper over on a rigid work surface you can freeze, such as transparent plastic or acrylic. Secure the parchment to the work surface with tape.
Insert a No. 1 circle pastry tip in a pastry bag and fill the bag with black buttercream icing. Close the top of the bag with a twist tie.
Draw the outline with the black buttercream. Place the work surface in the freezer every few minutes to keep the buttercream firm while you trace to prevent smearing.
Fill one pastry bag for each color you need with colored buttercream. Use a wide L-19 pastry tip for the colors you'll apply to large areas for ease of application.
Fill in the colors on the parchment paper, continuing to place the tracing in the freezer every few minutes to keep the buttercream from smearing. Apply background colors last.
Hold the work surface up to the light to identify weak spots. Use a fine paint or pastry brush to nudge the buttercream to the edges of the outline as needed. Fill in any bare spots as needed.
Apply the background using a color that blends with the color of the cake. Smooth the background with a narrow offset pastry spatula and freeze the buttercream picture overnight.
Frost the cake when you're ready to apply the transfer. Take the transfer out of the freezer and remove it from the work surface.
Position the frozen buttercream side of the parchment paper on the cake and gently press it into the icing. Peel the parchment paper from the frozen buttercream.
Moisten a thick, lint-free paper towel with cold water after the buttercream thaws on the cake, about 20 minutes. Gently wipe the lines from the piping process from the buttercream to create a smooth surface on the transfer.