Heat oven to 350. Line 13x9 dish with foil; spray with cooking spray. Mix first 4 ingredients until blended. (Mixture will be thick.) Press 1/2 cake mixture into pan.
Bake 10 minutes. Sprinkle half each of chocolate and nuts over crust. Top with peanut butter. Cover with layers of remaining cake mixture, chocolate and nuts. Press gently with spatula. Bake 15 minutes Cool 1 hour. Refrigerate 4 hours before cutting into bars.
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