For the batter:
10 tablespoons (1 1/4 sticks) unsalted butter, room temperature, plus more for pan
For the sugar coating:
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.