Preheat your oven to 350°F (175°C) while you prepare your 9-inch springform pan. Grease the pan thoroughly with butter or non-stick cooking spray, then line the bottom with parchment paper for easy removal.

In a medium bowl, combine ½ cup all-purpose flour, ½ cup light brown sugar, and 1 teaspoon ground cinnamon. Pour in ¼ cup of melted unsalted butter and mix until crumbly.

Peel and dice 2 cups of Granny Smith apples into small chunks, setting aside 1 cup for mixing into the batter and the remaining cup for layering on top.

In a large mixing bowl, whisk together 2 cups of all-purpose flour, 1 teaspoon ground cinnamon, 2 teaspoons baking powder, ½ teaspoon baking soda, and ½ teaspoon salt.

In a separate bowl, beat ½ cup unsalted butter and 1 cup light brown sugar together until light and fluffy. Add in 1 large egg, ½ cup unsweetened apple sauce, ½ cup full-fat sour cream, and 1 teaspoon pure vanilla extract.

Gradually add dry ingredients to the butter-sugar mixture, stirring gently until just combined. Fold in the first cup of diced apples.

Carefully spread the batter into the prepared springform pan, using a spatula to even out the top.

Mix the second cup of diced apples with 2 tablespoons of light brown sugar and 1 teaspoon of cinnamon. Sprinkle this mixture over the top of the cake batter.

Sprinkle the crumble mixture evenly over the layered apples.

Place the prepared pan in the preheated oven and bake for 50-60 minutes until a toothpick comes out clean.

Remove the cake from the oven and let it cool in the pan for about 45 minutes before removing the sides of the springform pan.

If desired, prepare icing by mixing 1 cup of powdered sugar with 2-3 tablespoons of milk until smooth and drizzle over the cooled cake.