Prep the pan: Heat oven to 350°F (175°C). Line an 8x8-inch or 9x9-inch baking pan with parchment or lightly grease.
Mash the bananas: In a large bowl, mash bananas until mostly smooth with a few small lumps.
Whisk wet ingredients: Add eggs, applesauce or Greek yogurt, milk, vanilla, and sweetener.
Whisk until combined.
Add dry ingredients: Stir in oats, protein powder, baking powder, baking soda, cinnamon, and salt. Fold just until no dry pockets remain. Batter will be thick.
Fold in extras: If using chocolate chips, nuts, or fruit, gently fold them in now.
Bake: Spread batter evenly in the pan.
Bake 25–35 minutes, until the center is set and a toothpick comes out clean or with a few moist crumbs.
Cool and slice: Let cool 10–15 minutes. Slice into 9–12 squares.
Serve: Enjoy warm or at room temperature. Great plain, or with a spoonful of yogurt and a drizzle of peanut butter.