PLACE morsels and 3/4 cup peanut butter in large, microwave-safe bowl. Microwave on HIGH (100%) power for 1 minute 30 seconds; stir. Microwave at additional 10- to 15-second intervals, stirring until smooth. Add sugar; stir vigorously until smooth.
PLACE cereal and chopped 100 Grand in large bowl. Add 1 cup melted chocolate mixture; stir until evenly coated. Place on ungreased baking sheet. Using small metal spatula, shape into 10-inch circle with slightly raised 1-inch-wide border. Pour remaining chocolate mixture in center of circle; spread to border.
PIPE concentric circles with decorator icing on top of chocolate mixture. Using wooden pick or tip of sharp knife, pull tip through icing from center to border. Refrigerate for 30 minutes or until firm. Cut into wedges.