Spray 12 regular-sized muffin cups with non-stick spray.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, nutmeg, cinnamon, and pumpkin pie spice.
In a small bowl, whisk together buttermilk and pumpkin puree.
In a large mixing bowl, beat butter and brown sugar until light and fluffy.
Beat in eggs and vanilla.
With mixer on low, add half of the flour mixture, then the pumpkin mixture, then remaining flour mixture, beating until just combined.
Divide batter evenly among muffin cups.
Bake at 350 degrees for about 25 minutes, or until a toothpick inserted near the center comes out clean.
Meanwhile, in a small bowl, combine granulated sugar and cinnamon.
Let muffins cool 10 minutes in pan on a wire rack.
Remove muffins from pan, dunk the tops (or the entire muffin) in the butter, then toss to coat in sugar mixture.
Serve warm.