Preheat the oven to 325 degrees Fahrenheit and spray an 8x4" loaf pan with cooking spray.
In a small bowl, whisk together the whole wheat pastry flour, protein powder, cinnamon, nutmeg, baking powder, baking soda, and salt.
In a large bowl, whisk together the applesauce, yogurt, erythritol, oil, molasses, egg whites, stevia extract, and vanilla extract.
Dump the dry ingredients over the wet ingredients and fold together until MOSTLY mixed through. Fold in the diced apples, being careful not to overmix.
Pour the batter into the prepared pan, sprinkle with the streusel, and bake for ~50 minutes, or until the surface springs back when tapped. Transfer the loaf to a wire cooling rack and let cool completely. Slice, serve, and enjoy! Store in a tightly sealed container in the fridge for up to 5 days.