Preheat the oven to 350°F (175°C). Line two 12-cup muffin pans with parchment liners.
Whisk dry ingredients: In a medium bowl, mix together flour, baking soda, baking powder, salt, and cinnamon.
Mix wet ingredients: In a large bowl, whisk together brown sugar, oil, eggs, and vanilla until smooth. Add mashed bananas and pumpkin purée. For a smoother texture, use an immersion blender to blend out banana chunks.
Combine: Fold the dry ingredients into the wet mixture until just combined. If the batter seems too loose, add a little extra flour. It should be thick and scoopable.
Add chocolate chips and fold gently.
Scoop and bake: Divide batter evenly into muffin cups, filling about ¾ full.
Bake for 20–25 minutes, or until a toothpick inserted in the center comes out mostly clean.
For tall, bakery-style domes: Start baking at 425°F (220°C) for 5 minutes, then reduce the temperature to 350°F (175°C) and bake for 14–16 more minutes.
Cool in the pan for 5 minutes, then move to a wire rack to finish cooling.