Mix together the oil, sugar, eggs, vanilla and pumpkin in a bowl. Set aside. In a separate bowl, mix together the flour, salt, baking soda, baking powder, cinnamon and pumpkin pie spice. Add the wet ingredients to the dry ingredients and mix just until combined.
Spoon the batter into muffin pans that have been sprayed with cooking spray. Divide the batter to make 18 muffins.
In a small bowl, cut the butter into the flour and sugar with a knife and/or fork. Mix until it resembles crumbs and the butter is in small pieces.
Divide the streusel between the 18 muffins, sprinkling it over the unbaked dough.
Bake the muffins for 13-15 minutes, until the cupcakes have risen and a toothpick inserted in the center comes out clean.
Allow the muffins to cool in the pan for 10 minutes, then remove them to a wire rack to cool completely.
Once the muffins are cool, use a sifting spoon to sift powdered sugar over top.
The calories shown are based on the recipe making 18 muffins, with 1 serving being 1 muffin. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.