Preheat your oven to 350 degrees F and grease your 8x8-inch baking pan with a nonstick cooking spray.

In a small bowl or cup, add vinegar and milk. Stir until combined and set aside for 5 to 10 minutes. This will make soured milk.

In a large bowl, add the flour, baking powder, baking soda, and cinnamon. Whisk until combined.

Make a well in the center of the dry ingredients and add the soured milk, vegetable oil, vanilla extract, applesauce, and sugar in it. Stir well until you no longer see dry ingredients. Some small clumps are fine.

Add the diced apples, brown sugar, flour, and cinnamon in a medium bowl and stir until the apples are coated well. Set aside.

In a small separate medium bowl, add the flour, sugar, and cinnamon. Whisk until combined.

Add the melted butter into the mixture and whisk until well incorporated.

Add about ⅔ of the batter to the bottom of the greased baking dish.

Spoon the apple filling evenly on top of the cake batter.

Add the remaining batter on top of the filling, carefully spreading the batter over all of the apples.

Add the topping on top of the batter spreading it out as evenly as possible.

Place in the preheated oven and bake for about 45-50 minutes, or until a toothpick poking into the center of the coffee cake comes out clean or with a few crumbs. The toothpick may also pick up some wetness from the apples or topping, so looking for a few moist, but fully baked crumbs, is ideal.

Let cool slightly, ice with icing if desired, and enjoy!