For topping: In a small bowl combine brown sugar, flour, walnuts, and powdered sugar. Cut chilled butter in small pieces and add to dry ingredients. Use fingers to incorporate butter, making a crumbly mixture. Refrigerate until ready to use.
For cake: Preheat oven to 350 degrees. Grease two loaf pans, approximately 8½ by 4½ inches each. In a large pan, combine pumpkin puree, Pumpkin Pie Spice, and salt. Cook over medium heat, stirring constantly until mixture is reduced to 1½ cups, about 8 min. Remove pan from heat and whisk in brown sugar, sour cream, and vegetable oil. Whisk in eggs, buttermilk, and orange juice until well-combined. In a separate bowl, whisk together flour, baking powder, and baking soda. Fold flour mixture into pumpkin mixture just until combined. Fold walnuts into batter. Divide batter between prepared pans. Sprinkle topping evenly over both loaves. Bake for 40 to 45 min., or until topping is golden brown and a toothpick inserted into the center comes out clean. Let loaves cool in pans for 15 min. before removing loaves to a wire rack to cool.