Bake and cool 8 x 2 in. round cake. Use spatula to icing cake smooth.
Transfer hamburger pattern to cake top with Piping Gel. Use tip 12 and light brown icing to pipe bottom of bun. Use tip 104 and green icing to pipe wavy line for lettuce. Use tip 12 and dark brown icing to pipe hamburger (smooth with finger dipped in cornstarch). Use tip 104 and red icing to pipe tomatoes. Use tip 12 and light brown icing to pipe top of bun. Smooth bun icing with spatula. Use tip 3 and red icing to pipe vertical zigzag lines about 1 in. apart on cake sides. Use tip 18 and yellow icing to pipe tight zigzag bottom border. Use tip 12 and white icing to pipe bead top border.