Prepare the Cake Batter:
Preheat your oven to 350°F (175°C). Grease and flour a 9×9-inch square baking pan or line with parchment paper.
In a medium bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt. Set aside.
In a large mixing bowl, cream softened butter and granulated sugar until light and fluffy (about 2-3 minutes).
Add eggs, one at a time, beating well after each addition. Stir in vanilla extract.
Gradually add the dry ingredients and sour cream, alternating between the two. Start with the dry ingredients, then add sour cream, mixing just until combined.
Gently fold in the chopped apples.

Prepare the Crumb Topping:
In a separate bowl, mix flour, cinnamon, salt, brown sugar, and granulated sugar.
Add cold, cubed butter and use a pastry cutter or your fingers to mix until the mixture resembles coarse crumbs with pea-sized butter pieces.

Assemble and Bake:
Pour the cake batter into the prepared pan and spread evenly. Sprinkle the crumb topping over the cake batter.
Bake for 55-65 minutes, or until the top is golden and a toothpick inserted into the center comes out clean or with a few moist crumbs.
Let the cake cool for 15-20 minutes before transferring it to a wire rack to cool completely.

Prepare the Cinnamon Apple Drizzle:
In a small saucepan, melt butter over medium heat. Stir in brown sugar, cinnamon, milk, and a pinch of salt.
Cook for 2-3 minutes, stirring occasionally, until the mixture is smooth and the sugar has dissolved.

Serve:
Once the cake has cooled, drizzle the warm cinnamon apple sauce over the top. Optionally, serve the drizzle on the side for guests to add as desired.