Pull out the egg, egg white, butter, and Greek yogurt to get to room temperature before starting the recipe.
Preheat oven to 350°. Grease a 9x5 inch loaf pan with non-fat cooking spray.
In a small bowl, combine 1/3 cup sugar and 2 teaspoons cinnamon then set aside.
In a medium, bowl combine bananas, yogurt, and vanilla extract. Mash bananas and mix all ingredients until well combined.
Sift together flour, baking powder, baking soda, and salt into a large bowl. If you don’t have a sifter, you can use a metal strainer/sieve!
Using a mixer, beat the butter and brown sugar until light and fluffy for about 3 minutes. Add the eggs one at a time and beat until fully combined for about 1 minute.
Add banana mixture and beat until well combined for about 30 seconds.
Add flour mixture slowly and beat until well combined for about 30 seconds. Add walnuts until well combined.
Pour half the batter into the loaf pan and spread out the batter until even.
Sprinkle half the cinnamon mix onto the batter.
Repeat with the remaining batter and cinnamon mix.
Draw a knife through the batter to make a marble effect.
Bake in the oven for 60 minutes, covering the pan loosely with foil after 30 minutes, until the top is golden brown and starting to crack.
Let the pan cool for 15 minutes on a metal rack.