150g bramley apples, peeled, cored and chopped into 1 cm pieces
100g blackberries, halved
Make the Base and Crumble Topping: Preheat the oven to 180 C / 160 C fan. Put the flour into a bowl with a pinch of salt. Add the butter, sugar and cinnamon and rub in with your fingertips until no visible lumps of butter remain and the mixture resembles coarse breadcrumbs.
Set aside 1/3 of the mixture and mix the egg into the remaining 2/3, mixing everything together well. Press this mixture into the base of a 30 x 23 cm baking tin with high sides. Bake in the oven for 15 minutes, until sandy and slightly golden then remove and allow to cool.
Make the Filling :Using an electric hand whisk to beat the cream cheese and sugar together. Add the sour cream, vanilla and custard powder and whisk briefly again then add the eggs and egg yolks and mix until smooth.
Put the tin into a deep oven tray or dish, to act as a water bath, and boil the kettle. Pour the cheesecake filling into the tin and cover the top of the tin loosely with foil, then put the whole thing into the oven. Pour boiling water into the water bath, so that it comes half-way up the cheesecake.
Bake the cheesecake for 35 minutes, then remove and scatter the apple pieces and blackberries over the top of the cheesecake. Top with the reserved crumble and place back into the oven to cook for a further 35 minutes until just set. Switch off the oven and crack the oven door but leave the cheesecake in there for 1 hour, so it cools slowly. Then remove the cake tin from the water bath and leave it to cool completely, in its tin. Remove from the tin and cut into bars to serve.