Mash the bananas in a large bowl until smooth. Stir in the melted butter, sugar, egg, and vanilla.
Sprinkle the baking soda over the mixture and stir. Add both flours and salt, stirring just until combined. Do not overmix.
Grease a 12-cup muffin pan or line with paper liners. Divide the batter evenly among the cups.
To bake on a camp stove: Place the muffin pan in a covered cast iron skillet or Dutch oven. Cook over low to medium-low heat for 18-22 minutes, or until a toothpick inserted in the center comes out clean. Rotate the pan halfway through for even baking.
To bake in a campfire Dutch oven: Use a tripod or grate to suspend the Dutch oven above a bed of coals. Place 8-10 coals on the lid. Bake for 20-25 minutes, checking at 20.
Let the muffins cool in the pan for 5 minutes, then turn out onto a cooling rack or clean towel. Enjoy warm or at room temperature.