PREHEAT oven to 350°F. Beat margarine and sugar in small bowl with electric mixer on medium speed until well blended. Add flour and walnuts; stir until mixture is well blended and crumbly. Remove 3/4 cup of the walnut mixture; set aside for later use. Press remaining walnut mixture firmly onto bottom of 15x10x1-inch baking pan. Bake 15 to 18 min. or until lightly browned.
MIX pumpkin, condensed milk, molasses, eggs, pumpkin pie spice and vanilla until well blended; pour over crust. Sprinkle with reserved walnut mixture.
BAKE an additional 30 to 35 min. or until pumpkin mixture is set. Cool in pan on wire rack. Drizzle with Powdered Sugar Glaze. Cut into 36 bars to serve. Store in tightly covered container in refrigerator.