Preheat your oven to 350°F and lightly grease a small oven-safe baking dish with a touch of ghee.
In a medium mixing bowl, mash the banana until very smooth, then whisk in the egg whites, 0.25 cup of the Greek yogurt, and vanilla extract.
Add the oat flour, vanilla protein powder, baking powder, and sea salt to the wet ingredients, stirring until the batter is well combined.
In a separate small ramekin, mix the ground cinnamon, coconut sugar, and melted ghee to create the cinnamon roll filling.
Pour half of the banana batter into the prepared baking dish, then dollop half of the cinnamon mixture on top.
Layer the remaining banana batter over the cinnamon, then finish with the remaining cinnamon mixture on the very top.
Use a knife or toothpick to gently swirl the cinnamon mixture into the batter for a marbled effect.
Bake for 22 to 25 minutes until the cake has risen and a toothpick inserted into the center comes out clean.
Allow the cake to cool for about 5 minutes before spreading the remaining 0.25 cup of Greek yogurt over the top as a protein-packed frosting.