Heat oven to 350°F (180°C) and fold a 7x11 inches (15x25 cm) baking pan with parchment paper.
Add ripe bananas to a large bowl and mash until smooth.
Add egg, maple syrup, almond butter, vanilla extract, cinnamon and salt. Using a whisk or fork, mix until smoothly combined.
Add oat flour, protein powder and baking powder to the wet ingredients. Using a spatula or fork, stir well until the dry ingredients are fully incorporated into the wet mixture. The batter should be smooth and free of lumps.
Protein powders absorb liquids differently. If your batter is too dry, add a splash of milk. If it's too runny instead, mix in a spoonful of flour.
Transfer the protein banana batter to the prepared baking pan, spreading it out evenly.
Bake in hot oven for 30 - 35 minutes, or until a toothpick inserted into the center comes out clean. Do not overbake them.
Allow banana protein bars to cool completely before frosting. In the meantime, prepare the healthy frosting: add all ingredients to a bowl and mix until combined.
Frost, then slice into 12 squares.