Preheat oven to 350° F (177° C).
Add ingredients for peanut butter icing to a medium bowl and mix with the whisk attachment on a hand mixer. Place bowl in refrigerator while you bake the muffins and let them cool (approximately 1 hour)
Mix mashed banana, peanut butter, honey, vanilla, yogurt and eggs using a hand mixer. Make sure that you use a full cup of mashed banana and measure that first.
Add oats to a food processor and blend to a fine powder. Add protein powder, cocoa powder, cinnamon, salt, baking soda, and baking powder and pulse to combine.
Pour dry ingredients into wet ingredients and mix in with a hand mixer while scraping the sides of the mixing bowl until just mixed together. Do not over mix!
Spray muffin tin with cooking spray or line with muffin wrappers. Spoon batter into prepared muffin tin, filling each cup a little over 3/4 of the way full.
Bake for 15 to 17 minutes until muffins are cooked through. Don't over bake or they will be dry! Test for doneness by inserting a toothpick in the center. Let muffins cool in pan for 10 minutes. Remove and rest on a wire rack for 20 - 30 minutes until cool.
Remove frosting from fridge and stir. Either spread on top of each muffin or use an icing bag to add frosting to each muffin. Store cupcakes in fridge until about 15 minutes before serving to keep the frosting from melting.