1.Preheat oven to 350 degrees F Or 180 degrees C.

2.Mix the artificial sweetener, yogurt and eggs together in a large bowl until well combined. Add applesauce, vanilla and 6 tablespoons melted butter; mix well again, until combine and set aside.

3.Add 1 cup all-purpose flour, whole-wheat flour, baking soda, apple pie spice and salt in a medium bowl. Add chopped apple and stir until the pieces are evenly coated. Add the flour-apple mixture to the egg mixture; mix it until just combines. Spoon into the prepared loaf pan.

4.Combine walnuts, brown sugar, cinnamon and the remaining 1 tablespoon each melted butter and all-purpose flour in a small bowl. After, sprinkle evenly on the batter in the pan.

5.Bake for 55 to 60 minutes. Cool in the pan for 10 minutes. After turn it out onto a wire rack; let cool completely, about 1 hour.