Mash the banana and mix with oats or granola. Add enough to get a thick, scoopable mixture—not too runny or too stiff, like a thick purée.

Add vanilla extract or another flavor of your choice (I used about 1 capful of vanilla).

Add the beaten egg and mix everything together.

Spoon the mixture onto a greased baking sheet, flattening each slightly. Use another spoon to help shape them. Make each cookie the size you like; I made 10 cookies using 1 tablespoon per cookie.