Preheat your oven to 350°F, and grease a muffin pan or line with paper liners.
In a large mixing bowl, beat the butter and brown sugar until fluffy, about 2 to 3 minutes.
½ cup unsalted butter, ⅔ cup light brown sugar
Add the pumpkin purée, egg, and vanilla. Mix until smooth and fully combined.
1 cup canned pumpkin purée, 1 large egg, 1 teaspoon vanilla extract
In a separate bowl, whisk together the flour, pumpkin pie spice, baking powder, baking soda, and salt.
1 cup all-purpose flour, 1 tablespoon pumpkin pie spice, 2 teaspoons baking powder, ½ teaspoon baking soda, ½ teaspoon salt
Add the dry ingredients to the wet mixture. Stir just until no streaks of flour remain. Make sure not to overmix.
Gently fold in the oats.
1 cup old-fashioned rolled oats
Spoon the batter into the prepared muffin tin, filling each cup about halfway to two-thirds full.
Bake for 18 to 22 minutes, or until a toothpick inserted in the center comes out clean.
Let the muffins cool in the pan for a few minutes, then move them to a wire rack to finish cooling.