Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease it well.
Mix the wet ingredients. In a large bowl, whisk the mashed bananas, pumpkin puree, eggs, sugars, oil, and vanilla until smooth and well combined.
Whisk the dry ingredients. In a separate bowl, whisk the flour, baking soda, baking powder, pumpkin pie spice, and salt until evenly mixed.
Combine gently. Add the dry ingredients to the wet.
Stir with a spatula just until no dry streaks remain. If using add-ins, fold them in now. Do not overmix.
Fill the cups. Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake. Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool. Let the muffins rest in the pan for 5 minutes, then transfer to a wire rack to cool.
Enjoy warm or at room temperature.