Preheat oven to 350F (175C) and grease and flour the bottom and 1" up the sides of a bread pan³. Set aside.
In a large bowl, whisk together flour, baking powder, pumpkin spice, cinnamon, baking soda, and salt, and set aside.
In a separate bowl, whisk together pumpkin puree, oil, and melted butter. Stir until well-combined and then stir in sugars.
Add buttermilk, eggs and vanilla extract and stir until combined.
Add dry ingredients to wet and gently fold together with a spatula or spoon until just combined. Do not over-mix, it will make your bread dense/dry.
Evenly spread batter into prepared pan. If you will not be using the glaze you might wish to sprinkle the batter generously with coarse sugar.
Transfer to 350F (175C) preheated oven and bake for 55-60 minutes or until a wooden skewer inserted into the center comes out clean (or preferably with a few moist crumbs).
Allow to cool for 15 minutes before inverting onto a cooling rack to cool completely before decorating with glaze (if using).
Whisk together powdered sugar, butter, vanilla extract and 2 teaspoons milk. If glaze is too stiff, add additional milk, one teaspoon at a time, until glaze falls in a ribbon from the whisk (if glaze is too runny, add more powdered sugar).
Drizzle over cooled bread, allow to cool and then slice and enjoy!