Preheat the oven to 350°F (175°C). Grease and flour a 9-inch square baking pan.

In a medium bowl, whisk together 2 cups flour, baking powder, baking soda, and salt.

In a large bowl, cream 1/2 cup softened butter and granulated sugar together until light and fluffy, approximately 3 minutes.

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

Add the flour mixture alternately with the sour cream to the creamed mixture, beginning and ending with the flour. Mix until just combined.

Gently fold in the diced apples using a spatula.

Spread the batter evenly into the prepared baking pan.

Prepare the topping by mixing 1/2 cup flour, brown sugar, and 1 teaspoon cinnamon in a small bowl. Cut in the cold butter using a pastry blender or fork until the mixture resembles coarse crumbs.

Sprinkle the topping evenly over the cake batter.

Bake for 40 to 45 minutes, or until a wooden pick inserted into the center comes out clean.

While the cake cools slightly, whisk together the powdered sugar, milk, and 1/4 teaspoon cinnamon to create the drizzle.

Drizzle the glaze over the warm cake before serving.