Heat the oven to 355F (180C) and line the wells of a muffin tray with muffin liners.
Add the mashed banana, maple syrup, and almond butter to a large bowl and mix well. Mix in the flour, baking powder, and baking soda. Divide the batter between the 6 muffin cups. Top each one with the chopped walnuts or almonds.
Bake for about 18-20 minutes in the middle of the oven, or until a toothpick comes out clean. Remove the muffins from the oven and cool completely before serving.
Notes
To make nut-free: sub the almond butter for a seed butter or wowbutter. Sub the chopped nuts for sunflower seeds or pumpkin seeds.