Preheat oven to 350º F. Spray a 9x13 baking dish with nonstick cooking spray. Set aside.
Add all dry ingredients through brown sugar to a large bowl. Whisk together until well-combined. Add in diced apples, applesauce, milk, and melted butter. Stir until well-combined. Pour batter into prepared 9x13 baking dish.
In a small dish, combine all of the ingredients for streusel topping, mixing together with a pastry blender, a fork or even with your fingers. Add to the top of the apple breakfast cake batter.
Bake cake in preheated oven for 40-45 minutes until a cake tester inserted in the center comes out clean. Allow to cool for about 15 minutes before serving.
Cover cake slices tightly with plastic wrap or store in an airtight container for up to 3 days.