Preheat oven to 350ºF. Spray a donut pan with nonstick spray. Set aside.

In a large bowl, whisk together the flour, baking powder, salt, pumpkin pie spice, cinnamon, and all spice. Set aside.

In a medium-sized bowl, combine the pumpkin puree, buttermilk, butter, eggs, vanilla, granulated sugar, and brown sugar and use an electric handheld or stand mixer on low speed to mix the ingredients together until fully incorporated. 

Add the combined wet ingredients into the dry ingredients and beat on low speed until no flour remains. Fill a ziplock or piping bag with batter and cut off a corner; pipe each donut cavity about 2/3 full. Bake at 350ºF for 8-10 minutes, or until a toothpick inserted comes out clean. Donuts can be carefully removed from the pan as soon as they finish baking. *Allow the donut pan to cool completely before baking the remaining donuts.

While the donuts are in the oven, prepare the topping. Melt one stick of unsalted butter in a medium-sized bowl and set aside. In a separate bowl, combine the granulated sugar, cinnamon, and pumpkin pie spice.

Once the donuts have cooled for about 10 minutes, dip each donut into the melted butter, allowing any excess butter to drip off. Then immediately place the donut in the sugar/spice mixture and use a spoon to completely cover the donut. Repeat for each donut.