Prep the pan and oven: Heat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease with oil spray.
Mash the bananas: In a large bowl, mash the bananas until mostly smooth with a few small lumps.
Add the wet ingredients: Whisk in the eggs, Greek yogurt, maple syrup or honey, and vanilla until well combined.
Combine the dry ingredients: In a separate bowl, whisk together almond flour, oat flour, protein powder, baking soda, baking powder, cinnamon, and salt.
Bring it together: Add the dry mixture to the wet mixture.
Stir gently with a spatula until just combined. If the batter looks overly thick or pasty, stir in 2–3 tablespoons of milk to loosen it. It should be thick but spoonable.
Fold in extras: Gently fold in chocolate chips, nuts, or berries if using.
Don’t overmix.
Fill the cups: Divide the batter evenly among the muffin cups, filling each about 3/4 full.
Bake: Bake for 16–20 minutes, or until the tops are set and lightly golden and a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool: Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. This helps them set and prevents sogginess.