Preheat your oven to 350°F (175°C). Grease a 9-inch springform pan and line the bottom with parchment paper.

Whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a medium bowl.

Cream the softened butter with granulated and brown sugars in a large bowl until light and fluffy, approximately 3-4 minutes.

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

Incorporate the sour cream and milk into the batter until smooth.

Gradually mix the dry ingredients into the wet mixture until just incorporated.

Fold in the diced apples with care to distribute the pieces evenly.

Make the streusel: In a separate bowl, combine the flour, brown sugar, and cinnamon. Cut in the cold butter until the mixture resembles coarse crumbs.

Pour the batter into the prepared pan, smoothing the top evenly with a spatula.

Sprinkle the streusel topping generously over the batter.

Bake for 45-55 minutes, or until a toothpick inserted in the center comes out clean.

Cool for 15 minutes in the pan before gently removing the sides. Let it cool completely on a wire rack.

Drizzle with caramel sauce before serving.