Preheat the oven to 350ºF. Spray a donut pan with non-stick cooking spray. Set aside.
In a medium bowl whisk together the pumpkin puree, brown sugar, vegetable oil, and milk.
In a smaller bowl combine the flour, baking powder, pumpkin pie spice, cinnamon, nutmeg and salt. Gradually add the dry ingredients to the wet ingredients, mixing with a spatula just until combined.
Transfer the batter to a piping bag or ziplock bag, cut off the corner. Pipe the batter into each donut cavity. Evenly divide the batter among the 6 donuts.
Bake for 15-16 minutes or until a toothpick inserted in the center comes out clean. Allow the donuts to cool in the pan for 10 minutes.
Flip the pan upside down to release the donuts onto a cooling rack. Allow the donuts to cool for at least 10 more minutes.
Melt the candy melts according to the package. You can add vegetable oil or shortening to help thin out the candy melts. Dip the top of each donut into the melted orange candy melts. Allow the donut to set for 1-2 minutes before sprinkling on the orange sanding sugar. Allow the candy melts to harden.
Meanwhile, make the buttercream. In a small mixing bowl, beat the butter until smooth. Add in the powdered sugar and kelly green food coloring, beat until combined. Transfer the buttercream to a piping bag fit with a small round piping tip.
Use a skewer or something similar to make a small hole on the top of the donut. Put a broken pretzel piece in the hole as the pumpkin stem.
Pipe the green buttercream onto the donut, making vines or leaves.