Preheat the oven to 350°F, and grease or line a muffin pan with paper liners.
In a large bowl, use a hand mixer to beat the butter and brown sugar on low speed until light and fluffy, about 3 minutes. Add the pumpkin puree, egg, and vanilla, and beat until well mixed. Set aside.
½ cup unsalted butter, ⅔ cup light brown sugar, 1 cup pumpkin puree, 1 large egg, 1 teaspoon pure vanilla extract
In a separate medium bowl, whisk together the flour, pumpkin pie spice, baking powder, baking soda, and salt. Add the dry ingredients to the wet ingredients, and mix on low speed just until no flour streaks remain. Gently fold in the rolled oats.
1 cup all-purpose flour, 1 tablespoon pumpkin pie spice, 2 teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon salt, 1 cup old-fashioned rolled oats
Fill each cavity of the muffin pan about ½ to ⅔ of the way full. Bake for 18 to 22 minutes or until a toothpick inserted into the center comes out clean.
Enjoy warm, and store any leftovers in an airtight container for up to 3 days.