Preheat your oven to 350°F (175°C) and lightly grease a mini loaf pan with avocado oil or line with parchment paper.
In a medium mixing bowl, mash the banana until very smooth with no large lumps remaining.
Whisk in the egg whites, Greek yogurt, lemon juice, and vanilla extract until the wet ingredients are fully combined.
Add the vanilla protein powder, coconut flour, lemon zest, baking powder, and sea salt to the bowl.
Stir gently until a thick batter forms, ensuring the coconut flour is fully hydrated and no dry clumps remain.
Pour the batter into the prepared mini loaf pan and smooth the top with a spatula.
Bake for 25 to 30 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Remove from the oven and allow the loaf to cool in the pan for at least 10 minutes before slicing and serving.