Pre-heat the oven to 350 degrees F, and line a baking sheet with parchment paper (or lightly grease it).
In a mixing bowl, mash the bananas (peel first!) with a fork until mostly smooth.
Mix in the protein powder until it’s fully combined and not clumpy. Stir in the oats. I like to add a pinch or two of salt and about 1/4 cup of dried fruit. Sometimes I make a smiley face on the top with chocolate chips because whose day doesn’t start off better when they see that on the plate!
Spoon mounds of the mixture onto the prepared baking sheet. (I use a large ice cream scoop and level it off for 8 evenly sized cookies.)
Use the back of a spoon to shape them into nice circles if you aren’t using an ice cream scoop, and then flatten to desired thickness as the cookies will not spread during baking. Bake for approximately 12 minutes, give or take a few minutes, depending on thickness of cookie and individual oven. (Jumbo cookies take 15-16 minutes in my oven.) The cookies should be just cooked through the middle.
Remove from the oven and cool. Once cool, transfer to an airtight container and store at room temperature for several days or refrigerate for extended freshness. These cookies freeze well, too.