Preheat oven to 425F. Lightly grease a 12-cup muffin tin. Set aside.
In a large bowl, whisk pumpkin, sugar, eggs, oil, buttermilk, and vanilla until smooth. In a separate large bowl, whisk flour, baking powder, salt, and spices. Gently fold flour mixture into liquid, stirring until just combined. (batter will be quite thick)
Divide muffin batter evenly between prepared pan and smooth tops. For the topping, combine sugar and cinnamon in a small bowl, then generously sprinkle over muffin batter.
Bake muffins at 425F 5 minutes then reduce heat to 375F and continue baking an additional 15 minutes, or until a toothpick tests done.
Cool muffins in pan 5 minutes before transferring to a wire cooling rack. Enjoy muffins warm.
Notes
Store muffins in an airtight container at room temperature up to 4 days.