Mix brown sugar and corn syrup in large microwavable bowl. Microwave on HIGH 2 minutes or until mixture comes to boil. Stir in 3/4 cup of the peanut butter until well blended. Add cereal; mix well.
Press cereal mixture firmly into 13×9-inch pan.
Microwave chocolate and remaining 1/4 cup peanut butter in medium microwavable bowl on HIGH 1 to 1-1/2 minutes or until chocolate is completely melted when stirred, stirring after 45 seconds. Spread evenly over cereal mixture.
Refrigerate 1 hour until chocolate layer is set. Cut into 36 squares.