Melt chocolate or white baking bar; cool. Stir in corn syrup until combined. Transfer mixture to a large sheet of parchment or waxed paper. Let stand at room temperature for about 6 hours or until dry.
Gently knead mixture for 10 to 15 strokes or until smooth and pliable. To make a lighter-colored chocolate, knead some white chocolate into dark chocolate. 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 leaves, shape a portion of the chocolate mixture into a ball. Flatten slightly; place between 2 sheets of parchment or waxed paper dusted with powdered sugar. Roll to 1/8-inch thickness. Using small hors d'oeuvre or cookie cutters, cut the chocolate mixture into leaf shapes. Carefully lift the cutouts from the waxed paper and place atop and around cake. If desired, place smaller leaves on top of the larger leaves.