Make the shortbread. Grease a 9x5 inch loaf tin and line it with non-stick baking paper, leaving a little overhang for easy removal.
In a mixing bowl, combine the flour, sugar and salt (if using). Add the vanilla, then the cubed butter and rub it into the flour till all the flour is moistened. Bring it together with your hands and it will form a soft dough.
Press the dough into the prepared tin, using your fingertips to spread it evenly and firmly into the pan. You can also use the bottom of a measuring cup to help spread it. Once done, place the tin in the fridge for 10 minutes. Preheat the oven to 175 C.
Bake the shortbread base for 10 to 12 minutes until barely browned. Set aside to cool.
Make the apple topping. Peel and core each apple, then slice (not too thin, not too thick; I do this while the base is chilling in step 3) and set aside. In a wide saucepan on low heat, melt the butter, then add the sugar and stir together, letting it cook for a minute. Add the apples and cinnamon and stir well to make sure each apple slice is well coated. Spread them out in a single layer and let them cook on each side for 2 to 3 minutes, until lightly caramelised. They will not have softened completely yet. Transfer to a bowl and set aside to cool.
Make the cheesecake filling. Preheat the oven to 160 C. Combine the cream cheese, yoghurt and cream in a bowl. Beat with a hand mixer on low speed for about 20 seconds, until smooth and creamy
Add the vanilla and sugar and beat gently again to combine. The sugar will loosen up the mixture considerably.
Finally, sprinkle the salt and cornflour into the bowl and beat briefly to incorporate it, or stir using a spatula. The batter is pourable but not super runny. Pour over the shortbread base and smoothen the top.
Now add the apples in a single layer, but overlapping them where necessary to fit all the pieces in (see photos below). Drizzle on any of the caramel left in the pan.
Bake for 45 to 50 minutes until the cheesecake layer appears set around the sides and you may see it puffing up in the space between the apples. The center will wobble a little but if the whole surface is still wobbly, bake for an additional 5 to 10 minutes.
Then turn off the oven and leave the pan in there for about 10 minutes, then let it cool at room temperature. The surface will fall slightly and the center will continue to cook and firm up on its own.
Once completely cooled, refrigerate the cheesecake at least 4 hours but preferably overnight and the next day, use the paper overhang to lift it out and slice into 8 bars. Store in the fridge in an airtight tin for 4 to 5 days and in the freezer for a month. Happy baking!