Preheat oven to 400 degrees. Oil muffin pan, or line with paper liners. Whisk together the flour, salt, and baking powder in a large bowl.
Whisk liquefied coconut oil, egg, milk, and maple syrup in a separate bowl until smooth. Remember that if you add coconut oil to cold ingredients it will turn solid. If you have trouble, just warm the bowl and batter in the microwave or over a pot of warm water. (I always make sure to use a microwave-safe bowl for just for this reason. You would think I would eventually learn this lesson.)
Stir the liquid ingredients into the flour mixture until moistened. Lightly fold in the mashed bananas and walnuts. Spoon the batter into the prepared muffin cups, filling them 2/3 full.
Bake muffins in the preheated oven until they rise and the tops are golden brown, about 16 minutes for a standard-sized muffin pan. A toothpick inserted into the center of a muffin should come out clean.