PREPARE cake mix according to package directions for 13 x 9-inch baking pan. Cut cake as shown in illustration.
ADD 3 to 4 drops blue & red food colouring to whipped topping to colour frosting purple. Using a small amount of frosting to hold pieces together, arrange cake on serving tray, as shown in photograph.
FROST cake with remaining whipped topping. Decorate with assorted candies.