1. Prepare the Crust:

• Preheat your oven to 180°C (350°F).
• Crush the digestive biscuits or graham crackers into fine crumbs using a food processor or by placing them in a sealed bag and crushing them with a rolling pin.
• Combine the crumbs with the melted butter until the mixture resembles wet sand.
• Press the mixture firmly into the base of a 23cm (9-inch) springform pan to form an even layer.
• Bake the crust for 10 minutes, then set aside to cool.

2. Prepare the Cinnamon-Spiced Apples:

• In a medium saucepan, melt the butter over medium heat.
• Add the diced apples, brown sugar, and ground cinnamon. Cook, stirring occasionally, until the apples have softened and the mixture is syrupy, about 10 minutes.
• Remove from heat and stir in the vanilla extract. Set aside to cool.

3. Prepare the Cheesecake Filling:

• In a large mixing bowl, beat the cream cheese and caster sugar together until smooth and creamy.
• Add the eggs one at a time, beating well after each addition.
• Mix in the vanilla extract, sour cream, and cornflour until fully incorporated.

4. Assemble the Cheesecake:

• Pour half of the cheesecake filling over the cooled crust.
• Spoon half of the cinnamon-spiced apples over the filling.
• Repeat with the remaining cheesecake filling and top with the remaining apples.

5. Prepare the Crumble Topping:

• In a mixing bowl, combine the plain flour, brown sugar, oats, ground cinnamon, and a pinch of flaky sea salt.
• Add the cold, cubed butter and rub it into the dry ingredients using your fingertips until the mixture resembles coarse crumbs.
• Sprinkle the crumble topping evenly over the assembled cheesecake.

6. Bake the Cheesecake:

• Place the springform pan on a baking sheet to catch any drips.
• Bake in the preheated oven for 50-60 minutes, or until the cheesecake is set around the edges but still slightly wobbly in the center.
• Turn off the oven, crack the door open, and let the cheesecake cool inside for 1 hour.
• Remove from the oven and allow to cool to room temperature, then refrigerate for at least 4 hours or overnight.

7. Prepare the Salted Caramel Sauce:

• In a medium saucepan, heat the caster sugar over medium heat, stirring constantly until it melts into a thick amber-colored liquid.
• Add the diced butter and stir until fully melted and combined.
• Slowly pour in the double cream while stirring continuously. Be cautious as the mixture will bubble vigorously.
• Allow the mixture to boil for 1 minute, then remove from heat and stir in the vanilla extract and flaky sea salt.
• Let the caramel sauce cool slightly before using.

8. Serve:

• Once the cheesecake is fully chilled, remove it from the springform pan.
• Drizzle the salted caramel sauce over the top before serving.
• Slice and enjoy!