PREPARE cake batter and bake in greased foil-lined 13x9-inch baking pan as directed on package. Cool completely. Cut cake into pieces and arrange on serving platter as shown in diagram; set aside.
PLACE chocolate and cream in microwaveable bowl. Microwave on MEDIUM 2 to 3 min. or until chocolate is completely melted and mixture is well blended, stirring after each min. Remove 1/4 cup of the chocolate mixture; place in small resealable plastic bag. Set aside. Add icing sugar to remaining chocolate mixture in bowl; beat with wire whisk until smooth and of spreading consistency. Spread onto cake.
DECORATE with candies as shown in photo. Cut 1/8-inch piece from one of the bottom corners of bag of melted chocolate. Use to pipe chocolate onto the cake for the "horse's mane."