Preheat the oven to 350F and line a muffin pan with 6 cupcake/muffin liners.
Then, add the banana to the mixing bowl and mash it until no large clumps remain. Next, add in the wet ingredients including the maple syrup, brown sugar, vanilla extract, oat milk, ground flax, and oil and gently whisk together. Once mixed, add in the dry ingredients including the flour, salt, baking powder, and baking soda, Whisk together just until a thick batter forms (try not to overmix).
Once a batter forms, gently fold in the walnuts. I like to set aside 3-4 chopped walnut pieces per muffin to add to the top right before baking.Then, add the batter to the 6 muffin tins in equal amounts until each tin is ¾ cup full with batter. At this point, place any walnuts you set aside on the top of each muffin.
Next, let the batter rest for 5 minutes. Letting it rest helps the baking soda & baking powder fully react and leads to fluffier muffins. After this rest period, add the muffins to the oven and bake for 18-22 minutes until cooked throughout and lightly browned on top.
Remove the muffins from the pan and let them cool on a wire rack. Enjoy!