Preheat the oven to 180°C. Peel the apples and remove the stalk but not the whole core. Very carefully, using a mandolin, thinly slice all of the apples and place in a
Bowl of cold water.
Grease a 20 centimeters springform cake tin with the butter. Mix together the demerara sugar and a tablespoon of the blitz biscuits, and sprinkle over the bottom of the tin, moving around the sides to cover.
Start laying up the apples in whatever order you fancy, but so there is a covered even layer. Once you have covered the bottom layer, mix all but 2 tablespoons of the blitz biscuit with the caster sugar, and sprinkle over the top of the apples. You can add a little cinnamon here if you fancy. Continue this process with the rest of the apples until they are all used up.
On the final layer, sprinkle over the cinnamon and top with an extra knob of sugar. Press the apples down lightly to ensure they are compact and then place in the oven for 30-40 minutes until the apples are soft and caramelised.
Whilst this is cooking, make the brittle by heating the sugar in a nonstick pan on a medium heat. Don't move the sugar around, however tempting. Only when it starts to bubble and all the sugar is turning golden, you can carefully swirl the pan to incorporate all the sugar. Quick pour into greaseproof paper or a silicone mat if you have one and sprinkle over the nuts and the reserve biscuits crumbs. Leave to cool and harden completely.
Whip the cream and stir through a bit of the blitz brittle if you like and any leftover nuts. Serve alongside the cake with the extra brittle. Delicious.