Preheat the oven to 350°F (180°C) and spray a muffin tray with non-stick spray. Set aside.
Add the all purpose flour, brown sugar, white sugar, baking soda, baking powder, salt, ground cinnamon and ground nutmeg to a mixing bowl and mix together.
Add the butter to a microwave safe bowl and melt it. Add the mashed banana, eggs and vanilla to the melted butter and mix together.
Pour the wet ingredients over the dry ingredients and mix into a batter.
Add the chopped walnuts and mix in.
Divide the batter evenly between the muffin partitions of your prepared muffin tray.
Add some more chopped walnuts directly to the top of the muffins.
Bake for 25-30 minutes at 350°F until beautifully browned on top and a toothpick inserted into the center of one of the muffins comes out clean.
Allow to cool for a few minutes before transferring the muffins to a wire cooling rack to cool completely.