Use a very dry blender to blend the oats into flour if you haven't yet. Stir together the oat flour, baking powder and baking soda, salt, and cacao powder in a large mixing bowl.
In a blender, combine the bananas, milk, flax seed, peanut butter (or other nut butter/oil), sugar, and apple cider vinegar.
Pour the banana liquid into the dry ingredients, and mix thoroughly. Stir in the chocolate chips and walnuts.
Spoon into a muffin pan (I also like these in a donut pan), and fill 3/4 full. Sprinkle with extra walnuts and chocolate chips if you'd like, or leave them blank on top for adding frosting.Bake standard muffins for 22-24 minutes, until tops are firm to the touch. Bake mini muffins for 10 minutes, and donuts about 18 minutes.