Make the crumble topping

• Whisk together the flour, light brown sugar, cinnamon and a pinch of salt.
• Add the cold diced butter and rub it into the flour mixture with your fingers until crumbly with some bigger chunks. Squeeze the mixture in your hand, then break it apart for larger crumbs.
• Chill in the fridge while preparing the filling and cake.

Cook the apple filling

• In a saucepan over medium heat, combine the apples, brown sugar, cinnamon, water, lemon juice, butter and salt. Stir until the butter melts.
• Cover and cook until the apples are softened.
• Stir in the cornstarch mixture and simmer until thickened. Set aside to cool slightly.

Preheat the oven and prepare the pan

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

Mix the dry ingredients

• In a medium bowl, whisk together the flour, baking powder and salt.

Cream the butter and sugars

• In a large bowl, beat the butter, granulated sugar and brown sugar until light and fluffy.

Add the eggs, vanilla and sour cream

• Add the eggs and mix until combined.
• Mix in the vanilla and sour cream until combined.

Finish the batter

• Add the dry ingredients and mix until just combined.
• Mix in the milk until just combined, being careful not to over-mix.

Assemble the cake

• Spread the batter evenly into the prepared pan.
• Spoon the apple filling on top, then sprinkle the crumble topping all over.

Bake the cake

• Bake until a toothpick inserted in the center comes out clean.
• If the top starts browning too much, cover it loosely with aluminum foil.

Cool and serve

• Cool at least 30 minutes, then run a knife around the edge of the pan.
• Slice and serve, optionally drizzled with caramel.