Preheat oven to 350°F (177°C). Spray a 24-count mini muffin pan with nonstick spray. Set aside.
In a large bowl, whisk the flour, baking powder, pumpkin pie spice, cinnamon, and salt together. In a medium bowl, whisk the melted butter and brown sugar together. Add the egg, vanilla extract, pumpkin puree, and milk and whisk to combine. Pour wet ingredients into the dry ingredients, then whisk until just combined. Avoid over-mixing. Batter is thick.
Using a small spoon, spoon the batter into the muffin tins, filling each about 2/3 full.
Bake for 12-14 or until a toothpick inserted in the center comes out clean or when lightly poked, the muffin bounces back. Remove from the oven and cool in the muffin pan for 10 minutes.
In a small bowl, mix the granulated sugar and cinnamon together. Use a spoon to help remove the warm muffins from the pan. Lightly dunk a muffin (they can still be a bit warm) in the melted butter. Submerge in the cinnamon-sugar to coat the surface generously. Set upright on cooling rack and repeat with remaining mini muffins.
Muffins taste best eaten on the same day. Cover and store leftover muffins at room temperature for up to 1 day, or in the refrigerator for up to 4 days. To help prevent the coating from turning mushy and wet, only loosely cover them, which will allow a little airflow.
