Preheat oven to 350°F with the rack in the middle position.
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 walnuts 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 350°F 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.