Preheat the oven to 375ºF (190ºC). Grease a 9-inch (23-centimeter) square baking dish with either butter or coconut oil.
In a medium mixing bowl, combine oats, cinnamon or pumpkin pie spice, baking powder, salt and optional protein powder. Whisk to combine.
In a smaller mixing bowl, combine milk, maple syrup, eggs, butter or coconut oil, and vanilla. Whisk until blended. (Note: if you're using coconut oil, sometimes it solidifies when it comes into contact with colder ingredients. If that happens, stick the bowl in the microwave and heat it up in 30-second increments until the coconut oil melts.)
Place the berries and banana on the bottom of the baking dish, gently mixing them. Cover the fruit with the dry oat mixture, then drizzle the wet ingredients over the oats. Make sure it soaks down through the oats and evenly. Gently press down the oats on top, using a spoon or fork.
Bake for about 45 minutes, until the top is golden brown. Remove from the oven and let cool for a few minutes before cutting into 8 portions! DELICIOUS!