Preheat Oven: Preheat your oven to 350°F (175°C). Grease an 8x8-inch baking dish or line it with parchment paper.
Mix Dry Ingredients: In a medium bowl, combine the rolled oats, baking powder, cinnamon, and salt. Stir to evenly distribute the ingredients.
Mix Wet Ingredients: In a separate large bowl, mash the ripe bananas. Add the peanut butter, milk, egg, honey (or maple syrup), and vanilla extract. Stir until the mixture is smooth and well combined.
Combine: Gradually add the dry ingredients to the wet ingredients, mixing until everything is combined. If desired, fold in chocolate chips or chopped nuts.
Pour Into Baking Dish: Pour the oatmeal mixture into the prepared baking dish, spreading it out evenly.
Bake: Bake in the preheated oven for 30-35 minutes, or until the top is golden and the oatmeal is set.
Cool and Serve: Allow the baked oatmeal to cool for a few minutes before slicing. Serve warm, and top with extra peanut butter, banana slices, or a drizzle of honey if desired.