Preheat the oven to 350 degrees Fahrenheit and spray a 9×5″ loaf pan with cooking spray.

In a small bowl, whisk together the whole wheat pastry flour, starch, protein powder, baking powder, baking soda, and salt.

In a stand mixer bowl fitted with a beater attachment, add the applesauce, yogurt, eggs, oil, erythritol, stevia, butter flavor, and zest.  Mix on low speed until smooth and even.

Dump the dry ingredients into the stand mixer bowl and mix until ALMOST combined.  Scrape down the sides of the bowl and mix one more time, JUST until the batter is even.

Pour batter into the prepared pan and bake for ~55 minutes, or until the surface forms a brown crust and springs back when tapped. Let cool completely in the pan, then slice and serve!  Keeps for ~1 week in a tightly sealed container in the fridge.