Preheat the oven to 350ºF (180ºC). Grease and flour a 24-serving mini muffin pan.* Do not use muffin liners. (The muffins need to form an outer crust. This prevents them from absorbing too much melted butter later in the recipe.)
In a medium-size bowl, whisk together the flour, baking powder, salt, cinnamon, ginger and nutmeg until well combined.
In a large bowl, whisk together the sugar, oil, egg and pumpkin purée until smooth and well combined. Stir in the flour mixture just until combined. The batter will be thick.
Transfer the batter to the prepared pan, filling the muffin cups about 2/3 full.
Bake until toothpick inserted into the muffins comes out clean, about 10-13 minutes. Do not over bake! Be diligent, since over baking can happen very quickly with mini muffins.
Let the muffins cool in pan for 5 minutes. Turn the muffins out and place on a cooling rack.
Topping
Whisk the sugar and cinnamon together in a shallow bowl until well combined.
Briefly dunk the warm muffins in melted butter, then roll in cinnamon sugar.