Preheat oven to 350ºF. Line a 12-cup muffin tin with paper liners or grease it lightly.
Mix dry ingredients:
In a large bowl, whisk together the flour, granulated sugar, baking soda, baking powder, salt, and spices.
Mix wet ingredients:
In a separate bowl, whisk together the pumpkin puree, brown sugar, buttermilk, oil, and egg.
Combine:
Pour the wet mixture into the dry ingredients. Use a spatula to gently fold until just combined. It’s okay if a few floury streaks remain—don't overmix.
Fill the muffin tin + bake:
Divide the batter evenly among the muffin cups, filling each about ¾ full.
Bake for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs or the internal temperature should be 190ºF on a meat thermometer. Let the muffins cool in the pan for about 5 minutes, then transfer them to a wire rack to cool completely.