Preheat oven to 425°F (218°C). Spray a 12-count muffin pan with nonstick spray or use cupcake liners.

Whisk the flour, oats, cinnamon, baking soda, baking powder, and salt together in a large bowl until combined. Set aside. In a medium bowl or in the bowl of your stand mixer, mash the bananas. Then whisk in the eggs, oil, maple syrup, milk, and vanilla together until combined. Pour the wet ingredients into the dry ingredients, stir a few times, then add the nuts. Fold everything together gently just until combined and no flour pockets remain.

Spoon the batter into liners, filling them all the way to the top. Top with oats and a light sprinkle of coconut sugar, if desired. Bake for 5 minutes at 425 then, keeping the muffins in the oven, reduce the oven temperature to 350°F (177°C). Bake for an additional 15-16 minutes or until a toothpick inserted in the center comes out clean. The total time these muffins take in the oven is about 20-21 minutes, give or take. (For mini muffins, bake 11-13 minutes at 350°F (177°C) the whole time.) Allow the muffins to cool for 5 minutes in the muffin pan, then transfer to a wire cooling rack to continue cooling.

Muffins stay fresh covered at room temperature for a few days, then transfer to the fridge for up to 1 week.