Preheat the oven to 350. Spray a mini muffin tin with cooking spray.
In a large bowl, whisk together the flour, baking powder, pumpkin pie spice, and salt. In a medium bowl, whisk together the pumpkin puree, sugar, egg, milk, and oil until well blended. Pour the wet ingredients into the dry ingredients, and mix until just combined.
Scoop the batter into the prepared muffin tin, filling the wells 3/4 full. Bake 11-13 minutes, until a toothpick inserted in the center comes out clean. Remove the muffins from the oven and let cool for 10-15 minutes.
In a small bowl, whisk together the sugar and pumpkin pie spice. Dip the muffins in the melted butter, coating all surfaces, then roll the muffins in the sugar mixture.