Preheat the oven to 350°F (180°C). Line a baking sheet with parchment paper.
In a medium bowl, mash the ripe banana with a fork until smooth. Add the egg and mix well until fully combined.
Add the almond flour, oats, brown sugar, baking powder, salt, and vanilla, and cinnamon. Stir gently until you get a thick, sticky dough. Fold in the chopped walnuts or pecans.
Let the dough rest for 5 minutes, then scoop about one tablespoon of dough per cookie. Place small mounds on the baking sheet and do not flatten them.
Bake for 15 to 17 minutes, until set and lightly golden. Let the cookies cool on the baking sheet for 10 minutes before moving them.
