Preheat your oven to 350°F (175°C). Grease an 8x8-inch baking dish or line it with parchment for easy removal.
In a large bowl, mash the bananas until mostly smooth.
Whisk in the eggs, Greek yogurt, maple syrup, and vanilla until creamy.
In the same bowl, add the oats, protein powder (if using), baking powder, baking soda, cinnamon, and salt. Stir until the oats are evenly coated and the batter is thick and cohesive.
Fold in any optional mix-ins like nuts, seeds, or chocolate chips.
Spread the batter into the prepared dish, smoothing the top. Let it sit for 5 minutes so the oats can hydrate slightly.
Bake for 28–35 minutes, or until the center is set and the top is lightly golden.
A toothpick should come out mostly clean with a few moist crumbs.
Cool for at least 15 minutes before slicing. It firms as it cools, making cleaner slices.