1. Coat the inside of an oval cake pan (see note) with cooking spray. Line with plastic wrap. Cut cake lengthwise into 3/4-inch slices; set aside.
2. Spoon and press ice cream in an even layer in prepared pan. Spoon sorbet over ice cream and smooth top. Place cake slices over ice cream and sorbet, trimming to fit. Discard any cake scraps. Cover with plastic wrap and freeze until very firm, about 4 hours.
3. In 2-cup glass measuring cup, combine chips and shortening. Microwave on 50 percent power 3 to 4 minutes, stirring every minute. Let mixture cool to luke-warm. (If chocolate is too hot, it will melt the ice cream; if it's too cold, it won't pour).
4. Meanwhile, transfer ice-cream cake (flat side down) to a wire rack set over a waxed-paper-lined jelly-roll pan. Remove plastic wrap and unmold. Starting at one end, pour melted chocolate in a steady stream over ice-cream cake. Touch up any holes with a small knife; work quickly, before chocolate sets. Transfer cake to a serving platter and freeze 15 minutes, to set coating.
5. Divide frosting into four or five bowls. Tint each bowlful a different pastel color with food coloring. Spoon frostings into small resealable bags and snip off tiny corners. Pipe a decorative design on cake with frostings. If using flowers, press onto frosting. Place in freezer until ready to serve. (Cake may be made up to three days ahead; cover lightly with plastic after frosting has set.) Use a sharp knife dipped into hot water to slice cake.