Preheat your oven to 350°F (175°C). If using a camp oven or Dutch oven, get your coals ready.
Grease a 12-cup muffin pan or line with paper liners. For a cast iron pan, use a generous swipe of butter or oil.
In a large bowl, mash the bananas with a fork until almost liquid with a few small lumps. Stir in the melted oil or butter, brown sugar, egg, and vanilla until smooth.
Add the flour, baking soda, baking powder, salt, and cinnamon. Stir with a wooden spoon just until no dry streaks remain — about 12 strokes. Do not overmix or the muffins will be tough.
Fold in the toasted walnuts gently. The batter will be thick and slightly lumpy.
Divide the batter evenly among the muffin cups, filling each about 3/4 full. Bake for 18–22 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool in the pan for 5 minutes, then transfer to a wire rack. Enjoy warm or at room temperature.