Prepare 2 standard loaf pans by lightly greasing and then dusting with flour.
In a large bowl, gently whisk dry ingredients (flour, baking powder, baking soda, salt, spices) and set aside.
In another large bowl (or the bowl of your electric mixer), add oil, sugar, eggs, vanilla and pumpkin. Blend well. Add water and blend until fully incorporated.
While slowly mixing, pour the dry ingredients in with the wet. Mix until just combined.
Pour batter into the prepared pans, filling pans about 2/3 full.
Prepare the streusel by mixing the brown sugar, flour, cinnamon and nuts (if desired) in a small bowl. Pour in the melted butter and stir gently until the mixture is combined, but some small lumps remain.
Sprinkle the cinnamon streusel topping on top of each loaf.
Bake loaves in a preheated oven for 50-60 minutes, or until a toothpick inserted in the center comes out clean.
This recipe may also be made into muffins. Bake muffins for 18-20 minutes.