Cover cookie sheet wit wax paper.

Place chocolate in a medium microwave-safe bowl. Microwave at 50% power for 1 minute; stir. If necessary, microwave at 50% power an additional 30 seconds at a time, stirring after each heating, until chocolate is melted and smooth when stirred.

Immediately place peanut butter chips and shortening in a second microwave-safe bowl. Microwave at 50% power for 1 minute; stir. If necessary, microwave at 50% power an additional 30 seconds at a time, stirring after each heating, until chocolate is melted and smooth when stirred; stir in peanuts.

Alternately spoon the chocolate and peanut butter mixtures onto the prepared tray; swirl with a knife for a marbled effect. Gently tap the tray on counter top to even out thickness or mixture. Refrigerate about 1 hour or until firm. Break into pieces. Store in a cool, dry place.