Preheat your oven to 350°F (175°C). Grease and flour a 9×9-inch square baking pan or line it with parchment paper.

In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, cinnamon, and salt. Set aside.

In a large mixing bowl, cream the softened butter and granulated sugar together until light and fluffy (about 2-3 minutes).

Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients in alternating batches with the sour cream, being careful not to overmix.

Gently fold in the chopped apples until evenly distributed throughout the batter.

In a separate bowl, combine the flour, cinnamon, salt, granulated sugar, and brown sugar. Add the cold, cubed butter and mix until it resembles coarse crumbs.

Evenly sprinkle the crumb mixture over the cake batter, covering the entire surface.

Place the cake in the preheated oven and bake for 55-65 minutes, or until golden brown and a toothpick comes out clean.

Let the cake cool in the pan for 15-20 minutes, then transfer it to a wire rack to cool completely.

In a small saucepan, melt the butter over medium heat. Stir in the brown sugar, cinnamon, milk, and a pinch of salt. Cook for 2-3 minutes until smooth.

Once the cake has cooled, drizzle the warm cinnamon apple sauce over the top.