Preheat the oven to 350 degrees F (175 degrees C). Grease a 12-cup muffin tin or line cups with paper liners.
Use a wooden spoon to mix butter into the mashed bananas in a large mixing bowl. Mix in sugar, egg, and vanilla extract. Sprinkle baking soda and salt over the mixture and blend in. Add all-purpose and whole wheat flours; mix just until combined. Gently mix in coconut, walnuts, cacao nibs, and flax seeds.
Spoon batter into the muffin tin.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 20 minutes.