Start by making the blackberry topping / filling. In a small saucepan combine the blackberries, water and sugar. Bring to a gentle simmer then gently crush the blackberries. Stir through the cornflour and leave to simmer for 10 minutes. Remove from the heat and transfer to a jug / jar. Pop this into the fridge to thicken and cool.
Lightly grease an 8 inch springform pan.
Crush the biscuits into a fine crumb (I used a food processor). Mix in the ground hazelnuts. Melt the butter and then pour this over the crumb mixture. Mix until combined.
Press this into the tin until smooth. Pop into the fridge.
For the filling whip the double cream until its thick and starts to form stiff peaks (do not over whip!).
Add in the cream cheese, icing sugar, hazelnut butter and chocolate - whisk until smooth. Spread half of this evenly over the biscuit base and swirl through some of the blackberry puree.
Cover with the remaining cheesecake mixture and then leave to set in the fridge overnight.
Once set cover with the remaining blackberry compote.
