Start by preheating your oven to 350°. Grease a 12-cup muffin tin and set aside.
In a medium-sized mixing bowl, mix together the dry ingredients - oats, protein powder, baking powder, cinnamon, and salt. Set the bowl aside.
Next, in a large mixing bowl, mash the bananas with a fork until there are no lumps. Then, add in the egg to the bowl and whisk it with the fork.
Next, add the rest of the wet ingredients - milk, maple syrup, and melted butter - and stir until the mixture is fairly smooth.
Then, add the dry mixture to the wet and stir well to incorporate everything.
Portion the batter evenly into the greased muffin tin.
Bake for 30-35 minutes until the tops of the muffins are golden brown and firm to the touch.
Remove the muffin tin from the oven and let the muffins cool in the pan for 15 minutes.
After 15 minutes, carefully remove the muffins from the pan and transfer them to a wire rack to cool completely.