Preheat the oven to 180°C (350°F) and prepare a muffin tin by lining it with paper liners or lightly greasing each cup, making sure the sides are coated so the muffins release easily after baking.
Place the ripe bananas in a large mixing bowl and mash them with a fork or masher until mostly smooth, leaving a few small lumps for texture and natural banana flavor.
Add the Greek yogurt, eggs, vanilla extract, and sweetener to the mashed bananas, then whisk until the mixture looks creamy, cohesive, and evenly combined with no visible streaks of egg or yogurt.
In a separate bowl, whisk together the oat flour, protein powder, baking powder, baking soda, cinnamon, and salt until well mixed, ensuring the leavening agents and protein powder are evenly distributed to prevent clumping.
Gradually add the dry ingredients to the wet mixture, folding gently with a spatula just until combined. Stop mixing as soon as you no longer see dry flour, as overmixing can make the muffins dense.
Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full so the muffins have room to rise without overflowing.
Place the muffin tin in the preheated oven and bake for 18–22 minutes, until the tops are set, lightly golden, and spring back gently when touched in the center.
Remove the muffins from the oven and let them cool in the tin for about 10 minutes, allowing them to set fully before carefully removing and serving.