Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or line it with parchment paper for easy removal.

In a large mixing bowl, whisk together the sifted flour, granulated sugar, baking powder, baking soda, and salt until well combined.

In another bowl, cream the softened butter until light and fluffy. This will help to incorporate air into the batter, ensuring a light texture.

Add the eggs, one at a time, mixing well after each addition. Then stir in the vanilla extract and sour cream until smooth.

Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Be careful not to overmix, as this can lead to a dense cake.

Fold in the diced apples, ensuring they are evenly distributed throughout the batter.

In a small bowl, combine the flour, brown sugar, melted butter, and cinnamon. Mix until crumbly. This will create a nice texture on top of the cake.

Pour the batter into the prepared cake pan, smoothing the top with a spatula. Sprinkle the streusel topping evenly over the batter.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean. This ensures the cake is fully cooked.

Once baked, remove from the oven and let it cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.

In a small bowl, whisk together the sifted powdered sugar, milk, and vanilla extract until smooth. Adjust the milk quantity to achieve your desired glaze consistency.

Once the cake is completely cool, drizzle the glaze over the top using a spoon or a piping bag for a decorative touch.