Preheat the oven. Set it to 350°F (175°C).
Line a baking sheet with parchment paper.
Mash the bananas. In a large bowl, mash the bananas until mostly smooth with a few small lumps.
Whisk in wet ingredients. Add the oat butter, brown sugar, and vanilla. Stir until well combined and creamy.
Combine dry ingredients. In a separate bowl, mix the rolled oats, oat flour, baking powder, cinnamon, and salt.
Bring the dough together. Add the dry mixture to the banana mixture. Stir until no dry spots remain.
Fold in any add-ins.
Rest the dough. Let it sit for 5–8 minutes. This allows the oats to hydrate, which helps the cookies hold their shape.
Portion and shape. Scoop heaping tablespoons of dough onto the baking sheet, spacing them about 2 inches apart. Gently press the tops to flatten to about 1/2 inch thick; they won’t spread much on their own.
Bake. Bake for 11–14 minutes, until the edges are set and lightly golden and the centers look just dry.
Cool. Let the cookies rest on the sheet for 5 minutes, then move them to a rack to cool completely.
They firm up as they cool.