Preheat oven to 340ºF (170°C) and line a baking sheet with parchment paper.
In a large bowl, use a fork or a potato masher to mash the ripe bananas until smooth.
Add oats and protein powder, then stir the mixture into a dough. If the dough is too wet, add more oats. If it's too dry, mix in a splash of milk, adjusting based on size and ripeness of the bananas.
Scoop the protein dough onto the lined baking tray, dividing it into 8 to 10 portions. Shape each into a ball, then gently flatten into cookies using your hands or the back of a fork.
Bake in hot oven for 8 to 10 minutes or until lightly golden on top. Don't worry if they are still a bit soft, they'll set further while cooling. Make sure you do not overbake the protein cookies or they'll get dry.