Preheat the oven to 350 degrees F. and butter a 9" square baking dish and set aside.
In a medium bowl, sift together flour, baking powder, salt, and cinnamon. Whisk to blend.
In a separate bowl, or in stand mixer bowl, blend together the melted butter, both sugars, eggs, vanilla extract, applesauce, and orange zest.
Add the dry ingredients to the wet ingredients and blend everything well.
Fold in the chopped apples, shredded zucchini, and walnuts.
Spread the mixture in prepared baking pan and bake in oven for 35-40 minutes or until toothpick inserted in center of cake comes out clean.
Cool thoroughly before frosting cake. Cut into 12 squares.