Preheat oven to 350F and line a baking sheet with parchment paper.
Make the cookie dough:
In a large bowl, mash the bananas until they are nearly smooth. Add the peanut butter, vanilla, and baking powder and mix well. I add it now instead of later with the dry ingredients, so it mixes nicely in the creamy base so every cookie has the same texture. Then, add all the other ingredients to the bowl and mix well until everything is combined and you have a thick dough.
Scoop 1 tablespoon-sized mounds onto the lined baking sheet. Then use your hands to shape them into a round cookie. They don't expand; these bake in place.
Bake:
Bake these oatmeal cookies in a preheated oven for 10-12 minutes. If you shape them on the thin side, check after 8 minutes. You know they're ready when the edges and most of the middles are set. These solidify as they cool. Let the cookies cool down on the baking sheet for 10 minutes.
Store:
Transfer to an airtight container and keep on the counter for 3 days or in the fridge for up to a week.
Notes
Nutritional values are based on a protein powder with 30g of protein per scoop and may vary depending on the brand used.