1. Preheat oven to 350 degrees. Butter and flour 12 standard muffin cups. Make batter: In a medium bowl, whisk together flour, baking powder, baking soda, salt, nutmeg, and allspice. In a small bowl, whisk together buttermilk and pumpkin puree. In a large bowl, using an electric mixer, beat butter and brown sugar until light and fluffy. Beat in eggs, one at a time, scraping down bowl as needed. With mixer on low, add flour mixture in three additions, alternating with two additions pumpkin mixture, and beat to combine.

2. Spoon 1/3 cup batter into each muffin cup and bake until a toothpick inserted in center of a muffin comes out clean, 30 minutes. Meanwhile, in a medium bowl, combine granulated sugar and cinnamon. Let muffins cool 10 minutes in pan on a wire rack. Working with one at a time, remove muffins from pan, brush all over with butter, then toss to coat in sugar mixture. Let muffins cool completely on a wire rack.