Pre-heat the oven to 400°F. Grease the cups of a 12-cup muffin pan with softened butter or nonstick cooking spray.
In a small mixing bowl, whisk together the pumpkin, sour cream (or Greek-style yogurt), eggs and butter. Set aside.
In a large mixing bowl, whisk together the remaining dry ingredients except the seeds. Add the wet ingredients to the dry ingredients and stir just to combine. Divide the batter between the cups and top each with the pumpkin seeds (if using).
Bake the muffins until they’re pale golden brown and a toothpick inserted into the center comes out clean, 18-20 minutes. Let the muffins cool in the pan for 10 minutes, then transfer to a wire rack to finish cooling.
Note: I recommend serving the muffins warm. If you’re baking them ahead of time, reheat them in a 250°F oven for about 10 minutes.