Preheat oven to 375°F. Coat twelve 2 1/2-inch muffin cups with cooking spray; set aside. In a large bowl, whisk together flour, cinnamon, baking powder, salt, and baking soda.
In a medium bowl, whisk together pumpkin and sugar. Whisk in eggs, one at a time, then the buttermilk, until well combined.
Pour pumpkin mixture into flour mixture. Stir briskly, with a spatula, until batter is combined. Batter will be thick. Do not overmix. Fill each muffin cup with about 1/3 cup of the batter. If desired, sprinkle each with the chopped pumpkin seeds. Bake until a toothpick inserted into the center of the muffins comes out clean, about 18 minutes. Remove muffins from tins while still warm. Repeat with remaining batter. Let cool or serve warm.
Cooled muffins may be refrigerated, in an airtight container, up to 3 days, or frozen, in plastic freezer bags, up to 1 month.
If you don't have buttermilk, you can make your own sour milk by using 1 1/2 tablespoons lemon juice plus enough reduced fat (2%) milk to make 1 1/2 cups. Let stand for 5 minutes before using.