Preheat & Prep: Preheat your oven to 350°F (175°C).
Mix Dry: In a large bowl, whisk together your flour, baking powder, baking soda, cinnamon, and optional protein powder.
Mix Wet: In a separate bowl, mash your bananas until smooth. Stir in the peanut butter, honey, eggs, milk, and vanilla until well combined.
Fold: Gently fold the wet ingredients into the dry. This process should be quick and gentle, as overworking the batter leads to dense muffins.
Bake: Spoon the batter into your prepared tin, filling each about 3/4 full. Bake for 16-19 minutes until a toothpick comes out clean.
Cool: Let them sit in the tin for 5 minutes before transferring to a wire rack.