Heat oven to 350 degrees. Coat a 9x5x3-inch loaf pan with nonstick cooking spray.
Mix flour, baking powder, pie spice and salt in small bowl. Beat egg and sugar in large bowl. Beat in pumpkin, oil and vanilla until smooth. Make a well in center of flour mixture. Add egg mixture. Stir just until moistened and batter comes together. Fold in walnuts. Spoon batter into prepared pan.
Mix butter, brown sugar and flour in small bowl until crumbly. Stir in walnuts. Sprinkle over batter.
Bake loaf at 350 degrees for 50 to 55 minutes, until wooden toothpick tests clean. Cool in pan on rack 10 minutes. Turn out onto rack to cool completely. Dust with confectioners' sugar; slice.