Mash the bananas in a large bowl until mostly smooth with a few lumps.
Stir in melted butter, brown sugar, egg, and vanilla until combined.
Add flour, baking soda, salt, and cinnamon. Mix just until no dry streaks remain — overmixing makes tough muffins.
Fold in walnuts or chocolate chips if using.
Grease a 12-cup muffin pan (or use paper liners). Divide batter evenly among cups.
Bake at 350°F (175°C) for 18–22 minutes, or until a toothpick inserted in the center comes out clean. If using a camp stove or fire, see tips below for even baking.
Cool in the pan for 5 minutes, then transfer to a rack. Let cool completely before packing for the trail.