LINE 13 x 9-inch baking pan with foil leaving an overhang on two sides. Lightly spray foil with nonstick cooking spray.
BEAT 1 1/4 cups peanut butter and butter in large mixer bowl until creamy. Gradually beat in 1 cup powdered sugar. Stir in remaining 1 cup powdered sugar, 3 cups pretzel pieces and 1/2 cup morsels. Press evenly into prepared baking pan. Smooth top with spatula.
MICROWAVE remaining 1 1/2 cups morsels and 3/4 cup peanut butter in medium, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 45 seconds; STIR. If necessary, microwave at additional 10- to 15-second intervals, stirring just until morsels are melted. Spread chocolate layer evenly over powdered sugar mixture. Sprinkle with the remaining pretzel pieces, pressing down lightly. Refrigerate for at least 3 hours or until firm. Lift from pan; peel off foil. Cut into 60 pieces. Store in covered container in refrigerator.