1. Heat the oven to 180C/160C fan/gas 3. Grease and line a 23cm loose-bottomed tin. Blend the broken biscuits to a fine crumb in a food processor. Add the melted butter, then briefly blend to combine. Press the mixture into the base of the prepared tin and bake for 10 mins. Set aside to cool.
2. On a lined baking tray, rub all the crumble ingredients together to make a rough mixture. Spread the crumble out evenly and bake for 10-15 mins until golden brown, turning halfway. Cool and store in an airtight container until ready to serve.
3. To make the filling, melt the Dr. Oetker White Chocolate chunks in a heatproof bowl placed over a simmering saucepan. Stir until melted, then remove and set aside to cool. In a heatproof jug, sprinkle the Dr. Oetker Powdered Gelatine over 2 tbsp hot water and whisk with a fork until dissolved. Set aside to cool slightly.
4. Using an electric whisk, beat the cream cheese until smooth, then add the cream, cooled white chocolate and Dr. Oetker Madagascan Vanilla Bean Paste. Whisk again to combine, then stir in the gelatine and pour the mixture over the cooled biscuit base, levelling with a spatula. Cover and chill overnight or until set.
5. For the blackberry sauce, press 300g blackberries through a fine sieve into a jug, then add 250ml of cold water, to form a purée. In a small saucepan, combine the Dr. Oetker Ground Arrowroot with a little of the purée to make a paste. Add the remaining purée and both sugars and gently bring to the boil, stirring until a glossy sauce. Leave to cool completely. To serve, drizzle the sauce over the cake and top with the crumble and remaining blackberries.