Preheat oven to 425°F. Grease a standard 12-cup muffin pan or line with baking cups.
In a large mixing bowl, microwave butter until melted. Add mashed bananas, sugar, and pumpkin and stir to combine.
Stir in the egg and vanilla until completely combined.
Add the gluten free flour blend, pumpkin pie spice, cinnamon, baking soda, and salt and stir until combined.
Divide the batter evenly among the prepared muffin cups.
Bake at 425° for 7 minutes, then reduce the oven temperature to 350° and continue baking for another 13-14 minutes. The muffins are done when a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Notes
Be sure to use pumpkin puree, not pumpkin pie filling. Pumpkin pie filling has added sugar in it and will make the muffins overly sweet.
When adding the dry ingredients to the wet ingredients, use a wooden spoon or spatula to stir everything together. Stirring or folding the mixture by hand will prevent overmixing the batter. Overmixing may lead to muffins that are tough.
Use a large scoop to fill the muffin cups with batter. This will keep the muffins similar in size and ensure even baking time.
Start the baking time at 425 degrees and then lower the temperature to 350 degrees after a few minutes. The higher temperature creates domed muffin tops and the lower temperature keeps the insides tender.