Preheat oven to 350F and grease a 9x9" baking dish with cooking spray.
In a large bowl, add the mashed banana, peanut butter, eggs, milk, maple syrup and espresso (if using). Whisk until combined.
Then, add in the cocoa powder and collagen (if using), whisk and let it dissolve.
Then, add in the remaining ingredients - oats, baking powder, salt and chocolate chips. Stir until combined.
Pour into the prepared baking dish and level out the top with a spatula. Add extra chocolate chips on top, if desired. Transfer to the oven and bake for about 40 minutes.
Immediately after removing from the oven, top with some flaky sea salt if desired). Let cool for about 10 minutes before slicing. Enjoy!