Preheat your oven to 180C (350F). Line a 9inch baking tin with baking paper and lightly grease with spray oil.
Blend together the milk, melted butter or coconut oil, banana, maple syrup, cocoa powder, and peanut butter, until smooth.
Place oats into a large bowl, pour over the wet ingredients and mix well to coat.
Place mixture into the prepared tin. Spread evenly and then bake for 20 - 25 minutes; until firm to the touch. Allow to cool before slicing. Store in the fridge for up to one week or freeze for up to one month.