Preheat oven to 350 degrees F. Butter and flour 12 standard muffin cups.
Make the Muffins: In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, ginger, nutmeg and cloves; set aside. In a small bowl, whisk together the buttermilk and pumpkin; set aside.
Beat the butter and brown sugar on medium speed until light and fluffy, about 3 minutes. Beat in the eggs, one at a time, scraping the sides of the bowl as needed. Reduce the mixer speed to low, then add the flour mixture in three additions, alternating with two additions of the pumpkin mixture, beating until barely combined. Turn off the mixer and fold the batter a few times with a rubber spatula to catch any pockets of flour that weren't incorporated.
Divide the batter evenly between the muffins cups (about a heaping 1/3 cup batter). Bake until a toothpick inserted in center of a muffin comes out clean, about 30 minutes. Let the muffins cool on a wire rack for 10 minutes.
Make the Cinnamon-Sugar Coating: While the muffins are resting, combine the sugar and cinnamon in a shallow bowl. Working with one muffin at a time, remove the muffins from the pan, brush all over with the melted butter, then toss to coat in the cinnamon-sugar mixture. Return to the wire rack and let cool completely.