Preheat your oven to 350°F and lightly grease a small mini-loaf pan with avocado oil or line with parchment paper.
In a medium mixing bowl, mash the half banana with a fork until it reaches a smooth consistency.
Add the liquid egg whites, nonfat Greek yogurt, and vanilla extract to the banana, whisking until the wet ingredients are fully combined.
Sift in the vanilla protein powder, coconut flour, baking powder, cinnamon, and sea salt, stirring gently until no dry clumps remain.
Roughly chop the walnuts and fold them into the batter, reserving a small amount to sprinkle on top.
Transfer the batter into the prepared loaf pan and smooth the top with a spatula.
Bake for 25 to 30 minutes, or until the loaf is golden brown and a toothpick inserted into the center comes out clean.
Allow the bread to cool in the pan for 10 minutes before transferring to a wire rack to cool completely before slicing.