Line 13 x 9-inch baking dish with aluminum foil so that foil extends about 2 inches over sides of dish; spray with cooking spray.
In large saucepot, melt marshmallows, ½ cup peanut butter and butter over medium-low heat; remove from heat and fold in cereal. Firmly press marshmallow mixture into even layer in prepared dish.
In small microwave-safe bowl, heat chocolate chips and remaining 2 Tbsp peanut butter in microwave oven on high 1 minute 30 seconds or until melted, stirring every 30 seconds. Pour chocolate mixture over cereal bar; spread into even layer and refrigerate 2 hours.
Using overhanging sides of foil, transfer cereal bar to cutting board and remove foil; cut into 16 bars.
