LINE 13x9-inch pan with foil, with ends of foil extending over sides. Spray foil with cooking spray. Melt caramels with milk in saucepan on low heat, stirring until caramels are completely melted and mixture is blended. Set aside.
BRING marshmallows, sugar, butter and evaporated milk to full rolling boil in large saucepan on medium heat, stirring constantly. Cook 5 min., stirring constantly. Add chocolate; stir until melted. Add vanilla and nuts; mix well.
POUR chocolate mixture into prepared pan; spread to cover bottom of pan. Pour caramel mixture over chocolate layer, re-warming caramel if necessary to pour; swirl with knife several times for marble effect.
REFRIGERATE several hours or until firm. Use foil handles to lift fudge from pan before cutting into squares.