LINE 13x9-inch pan with foil; spray with cooking spray. Beat first 4 ingredients with mixer until well blended. (Dough will be thick.) Press half onto bottom of prepared pan. Bake 10 min.
SPRINKLE half each of the chopped chocolate and nuts over crust. Microwave peanut butter in small microwaveable bowl on HIGH 1 min., stirring after 30 sec.; spread over nuts.
ROLL remaining dough into 13x9-inch rectangle between 2 sheets of waxed paper. Peel back top layer of waxed paper; flip dough onto top of dessert. Remove remaining waxed paper. Sprinkle with remaining chocolate and nuts; press gently into dough with spatula.
BAKE 15 min. Cool 1 hour. Refrigerate 4 hours before cutting into bars