Start with a purchased (or homemade) 2-layer chocolate cake frosted with purchased or homemade chocolate frosting. Use leaf, heart, star or other desired shape cookie cutters or stencils to create a decorative pattern around the top edge of the cake using shapes cut from light and/or dark chocolate clay. Or, set the cookie cutters or stencils on top of the cake and sprinkle assorted colored sugars in the cutters or stencils to make colored designs on the cake.
In a small saucepan melt 6 ounces semisweet chocolate or milk chocolate over low heat, stirring frequently; cool slightly. Stir in 3 tablespoons light-colored corn syrup until combined (do not overmix). Transfer mixture to a large sheet of waxed paper. Let stand at room temperature for 6 hours or overnight or until dry. Gently knead mixture for 10 to 15 strokes or until smooth and pliable. If mixture is too soft, chill in the refrigerator about 15 minutes or until easy to handle. Or, if desired, knead in enough powdered sugar to make the mixture stiff. Store unused chocolate in a sealed plastic bag at room temperature for 3 to 4 weeks. (It will stiffen during storage. Before using, knead the mixture until it is pliable.) To make shapes, shape a portion of the chocolate mixture into a ball. Flatten slightly; place between 2 sheets of waxed paper dusted lightly with powdered sugar. Roll to 1/8-inch thickness. Using small hors d'oeuvre or cookie cutters, cut the chocolate mixture into desired shapes. Carefully lift the cutouts from the waxed paper; sprinkle with colored decorative sugar, if desired, pressing lightly into the shape. Designs may be made on the shapes using a sharp knife. Smaller shapes may be placed on top of the larger shapes.