Preheat oven to 350F with the rack in the middle position.
Toast your coconut if it is not toasted already by spreading it out in a thin layer on a baking sheet and toast in the oven, stirring every couple minutes, until the coconut is starting to turn a light golden brown. Approximately 7-10 minutes.
While the coconut is toasting, start making the bread. In a large bowl, mix together the flour, sugar, baking soda, salt, and cinnamon.
In a medium bowl, combine the mashed banana, butter, eggs, yogurt, and vanilla extract.
Add the coconut and walnuts (if using) to the dry ingredients and stir to combine.
Generously coat a 9 by 5 inch loaf pan with nonstick spray.
Gently fold the wet mixture into the dry mixture until it is just combined. Some chunks and streaks is okay, and it will be thick.
Spread the batter into the loaf pan and smooth the top.
Bake in the oven at 350F until cooked through, approximately 50-60 minutes. A toothpick or cake pick inserted in the middle should come out with just a few crumbs.
Remove from oven. Let the bread sit in the pan for 10 minutes, then flip out and let cool on a cooling rack complete, approximately 1 hour.