Preheat oven to 190°C/170°C fan. Line a standard loaf tin with greaseproof paper.
Shortbread layer Cut the butter into cubes and rub it into the flour until the mixture resembles breadcrumbs.
Fork through the 50g caster sugar.
Pour ⅔ mixture into your loaf tin. It will be crumbly, but just press it evenly across the base of the loaf tin. Fork a few holes into the shortbread base.
Place in the oven for 15-20 minutes until the edges are beginning to brown.
While the shortbread is baking, let's continue with the other layers.
Apple layer Peel and dice up your apples. In a bowl mix your diced apples, cornflour (or flour), brown sugar (or whatever sugar you're using), cinnamon and vanilla until evenly coated. Set aside.
Streusel layer With the remaining ⅓ shortbread mixture, add your demerara sugar.
Spread the diced apples over the baked shortbread base. Sprinkle the crumble mixture across the surface.
Put back in your oven for 35-40 minutes until the streusel topping is baked and brown.
Leave cool for 20 minutes in the tin. I then put mine in the fridge for an hour to set firm before slicing.
Cut into squares and enjoy!