Preheat oven to 350°F. Line a cookie sheet with parchment paper or a silicone baking mat*.
Add mashed banana, melted butter, ground flax, sugar, and vanilla to a bowl. Mix until well combined.
Add oats, walnuts, flour, baking powder (sifted if clumpy), and cinnamon. Stir well until a dough forms. If using add-ins like raisins or chocolate chips, stir those in, too.
Scoop about 2 tbsp of dough and roll into a ball. Place on cookie sheet and slightly flatten. Repeat with remaining dough, leaving at least ½ an inch of space between each cookie (they will spread a little).
Bake 15 minutes. Let the cookies rest on the cookie sheet for 3 minutes, then transfer them to a wire rack to cool completely.
Notes
VEGAN BUTTER: Use soy-free butter if needed. Vegan butter is often salted, so I didn't add any extra salt to this recipe. If you're using unsalted butter or coconut oil, you can add a pinch of salt to the dry ingredients.GROUND WALNUTS: You can grind your own walnuts in a food processor/blender. Feel free to swap with other nuts/seeds such as almonds, pecans, sunflower seeds, etc. Or you can also use almond flour/meal.GLUTEN-FREE OPTION: Swap the whole wheat flour for all purpose gluten-free flour or buckwheat flour. Also make sure your oats are certified gluten-free.