Heat oven to 350°. Grease 9×13 pan. In large bowl, combine cake mix and margarine at low speed until crumbly. Reserve 1/2 cup crumbs for topping. Press remaining crumbs in bottom of prepared pan. In same large bowl, beat ground pecans, powdered, vanilla extract and egg whites on highest speed for 4 minutes. Pour evenly over crumbs. Combine 1/2 cup crumbs and chopped pecans, and sprinkle over egg white mixture. Bake at 350° for 20 to 30 minutes or until light golden brown. Cool completely; cut into bars. Yields 36 bars.