For the topping, stir together the brown sugar, flour, and pumpkin pie spice. Using 2 forks, cut in the butter until the mixture is crumbly. Set aside.
Muffins
Preheat the oven to 375 degrees F. Prepare a muffin pan with cupcake liners and spritz with nonstick cooking spray.
In a large bowl, whisk together the dry ingredients: flour, brown sugar, baking powder, pumpkin pie spice, salt, and baking soda.
In a separate bowl, whisk together the wet ingredients: egg, pumpkin, milk, and oil.
Stir the wet ingredients into the dry ingredients just until combined.
Add the oats and optional mix-ins if using. Stir gently to combine.
Divide the batter into the 12 muffin cups, filling each about ¾ full.
Spoon about 2 teaspoons of streusel topping over the top of each muffin.
Bake for 20 minutes or until a toothpick inserted in the middle of the muffins comes out clean.
Cool the muffin tin on a wire rack for 10 minutes. Transfer muffins to a wire rack to cool completely.
Notes
Do not use pumpkin pie filling.
Store on the counter for up to 2 days or in the refrigerator for up to 5 days.
Can be stored in the freezer - wrap in plastic wrap then place in a ziptop plastic bag for up to 3 months.
Nutritional information does not include optional ingredients.