Preheat oven to 350°F. Spray a standard 12-cup muffin tin with non-stick spray and set aside.
In a large mixing bowl, whisk together the flour, brown sugar, baking powder, baking soda, salt, and cinnamon.
Stir together the ground flax and water to create a flax “egg.” Set aside for a few minutes to thicken.
Add 3 1/2 bananas, non-dairy milk, and butter to a blender and blend until smooth. (Slice the other half of the banana and set it aside for topping the muffins with.) Mix into the dry ingredients.
Stir in the flax egg, vanilla, and apple cider vinegar just until combined.
Fold in chocolate chunks and walnuts, setting some aside to top the muffins with.
Divide the batter evenly between the 12 cups of the pan and top with banana slices and remaining chocolate chunks and walnuts.
Bake for 30 minutes, or until a toothpick inserted into the muffins comes out clean.
Allow muffins to cool in pan for 5-10 minutes before transferring to a wire rack to cool completely.