Preheat oven to 350. Butter and flour a standard 12 cup muffin pan.
Whisk together dry ingredients in a medium bowl. Whisk pumpkin, buttermilk, and vanilla in a small bowl.
Cut butter into tablespoons and beat until smooth. Add brown sugar and cream until light and fluffy, about five minutes. Beat in eggs, one at a time, until blended, scraping down the bowl as needed.
On low speed, alternate adding flour and pumpkin, beginning and ending with flour in 3 batches, until just combined. With a spatula, give the batter a couple of good folds to ensure all of the flour and pumpkin is incorporated.
Divide the batter between the muffin cups. Bake 28-32 minutes, or until an inserted toothpick comes out clean. Cool in pan on wire rack for 10 minutes.*
Combine granulated sugar and cinnamon in small bowl. Melt butter and brush over warm muffins. Roll muffins in cinnamon sugar.
*To freeze, cool muffins completely, then freeze up to 3 months. Reheat about 10 minutes in 350* oven, then brush with butter and roll in cinnamon sugar.