Preheat oven to 400°. Use muffin cups to line muffin tins or use cooking spray to lightly grease the pans. Prepare streusel topping by combining all the dry ingredients together first. Then work the butter into the dry ingredients with your fingers until evenly mixed and a crumbly consistency in reached.
In a large mixing bowl, combine the pumpkin, oil, sugar, molasses, vanilla and eggs until well blended. Combine flour, walnuts, and the remaining dry ingredients in another bowl and mix until well blended. Stir flour into the pumpkin mixture until well incorporated. Divide batter evenly in the tins. Sprinkle streusel topping over each one. Bake for 14-16 minutes or until toothpick, inserted in the center, comes out clean.
Cool on wire racks in the pans for 10 minutes. Remove from pans and continue to cool on racks until cooled completely. Store loosely covered.