Preheat oven to 350 degrees F. Prepare a piping bag fitted with a small round top. Lightly grease mini donut pan with cooking spray (Pam, Baker’s Joy). Set aside.
In the bowl of a stand mixer, cream together butter and brown sugar until light and fluffy. Beat in egg. Mix in pumpkin puree until combined.
Whisk together flour, baking powder, baking soda, salt, and spices. In another bowl, combine buttermilk and pumpkin puree.
With mixer on low, add dry mixture in three additions, alternating with pumpkin mixture. Be sure to start and end with the dry mixture. Mix until combined.
Fill prepared piping bag with batter. Pipe batter into prepared donut pans, filling mold a little over half full. Bake for 10-12 minutes. Let cool in pan for 2 minutes before unmolding. Let donuts cool for 5 minutes on rack before coating in cinnamon sugar
Cinnamon Sugar Coating: In a small bowl, whisk together sugar and cinnamon. Set aside. In another bowl, keep melted butter warm
Working with one donut at a time, dip donuts into melted butter. Toss to coat in cinnamon sugar mixture. Let donuts cool completely on wire rack.