In a large mixing bowl, combine the pumpkin, bran cereal, raisins, butter or melted coconut oil, and sugars. Stir in the eggs and the buttermilk.
In another bowl, whisk together the flour, cinnamon, nutmeg, baking soda, baking powder, and salt. Add to the pumpkin mixture and stir to just combine.
Cover the bowl and refrigerate for at least three hours or overnight.
Preheat the oven to 375 degrees.
Grease or line a 12-cup muffin tin and scoop the batter into the pan. I like to use a large ice cream scoop. Cups will be rather full but they will not overflow.
Sprinkle the tops with a little coarse sugar, if desired, then bake for 23-28 minutes or until the muffins pass the toothpick test. (Check a little early as all ovens vary.)
Allow muffins to cool for 5 minutes in the pan, and then remove to rack to cool completely.
Store leftovers in refrigerator and rewarm gently in microwave, if desired.