With mixer, beat margarine and sugar until creamy. Stir in flour and walnuts until mixture is crumbly. Reserve 3/4 cup crumb mixture for topping. Press remaining crumb mixture on bottom of 15-1/2 x 10-1/2 x 1" baking pan. Bake at 350°F for 15-18 minutes or until lightly browned.
Blend pumpkin, condensed milk, molasses, eggs, pumpkin pie spice and vanilla; pour over crust. Sprinkle with reserved crumb mixture. Bake 30-35 minutes more or until set. Cool in pan on wire rack.
Drizzle with powdered sugar glaze, if desired. Cut into 2-1/2 x 1-1/2" bars. Store covered in refrigerator.