Preheat your oven to 150°C (300°F) and grease your baking dish with a little butter or oil. Blend the oats in a blender to create oat flour.
Mix together the oat flour, protein powder, and cinnamon.
In a separate bowl mash half the banana (reserving the other half for toppings) and add the milk.
Add the dry ingredients to the banana and milk mixture and stir together until fully combined.
Pour half the mixture into your baking dish and spoon little drops of peanut butter evenly around the dish. Pour the rest of the mixture on top.
Bake for approx. 10 to 15 minutes depending on your dish and oven. To check for done-ness gently tap the top, if it's too liquidy give it another minute and check again.
Remove from the oven and top with the rest of the banana, and any other toppings you fancy.