Preheat oven to 425°. Line a 12-count muffin tin with paper liners.
In a large bowl, microwave 1/4 cup butter until melted (about 15-20 seconds). Add 2 very ripe bananas and mash them together with the butter until smooth.
Stir in 1 cup sugar and 3/4 cup canned pumpkin until fully combined.
Add 1 large egg and 1 teaspoon vanilla extract to the mixture and stir well.
Sprinkle in 1 1/2 teaspoons pumpkin spice, 1 teaspoon cinnamon, 1 1/2 cups all-purpose flour, 1/2 teaspoon salt, and 1 teaspoon baking soda. Mix gently until no dry spots remain—do not overmix.
Spoon the batter evenly into the prepared muffin tin, filling each cup about 3/4 full.
Bake at 425°F for 5 minutes. Then, without opening the oven, reduce the temperature to 350°F (175°C) and bake for an additional 15-16 minutes, or until a toothpick inserted in the center comes out clean. The initial high heat helps the muffins rise and develop a nicely rounded top.
Remove muffins from the oven and let cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.
