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

In a medium bowl, whisk together 1 ½ cups flour, baking powder, baking soda, ½ teaspoon cinnamon, and ¼ teaspoon salt.

In a large bowl, use an electric mixer to cream ¾ cup butter and 1 cup sugar until light and fluffy, about 2–3 minutes. Beat in the eggs one at a time, then stir in 1 teaspoon vanilla extract.

Add the dry ingredients in three batches, alternating with ½ cup sour cream, starting and ending with the dry ingredients. Mix gently to avoid a dense cake.

Fold in 2 chopped apples, distributing evenly without mashing.

In a separate bowl, combine 1 cup flour, 1 teaspoon cinnamon, ¼ teaspoon salt, ½ cup brown sugar, and ¼ cup granulated sugar. Cut in ½ cup cold butter cubes with a pastry cutter until the mixture resembles coarse crumbs.

Evenly sprinkle the crumb topping over the apple batter.

Bake for 55–65 minutes, until the top is golden and a toothpick comes out clean. Cool in the pan 15–20 minutes, then transfer to a wire rack.

For the drizzle, melt 1 tablespoon butter in a small saucepan over medium heat. Stir in ½ cup brown sugar, 1 teaspoon cinnamon, 2 tablespoons milk, and a pinch of salt. Cook 2–3 minutes until smooth, then drizzle warm over the cooled cake.