Blackberry and sour cream cheesecake
For the sauce * 150g fresh or frozen blackberries * 2 tbsp caster sugar * 1 tsp cornflour
For the cheesecake * 200g ginger nut biscuits
* 70g unsalted butter, melted * a pinch of salt * 750g full-fat cream cheese, at room temperature * 300g sour cream, at room temperature * 5 level tbsp cornflour
* 175g caster sugar * Zest of ½ a lemon, finely grated
* 1 tsp vanilla bean paste or extract * 5 large free-range eggs, at room temperature, plus 1 egg white * Start with the blackberry sauce. Put the blackberries and sugar in a small pan with a tablespoon of water. Heat gently, stirring, for 5 minutes, until the berries begin to soften. Purée the berries in the pan using a hand-held stick blender or in a blender. Mix the cornflour with a tablespoon of water to make a paste and combine with the blackberry purée in the pan. Bring to the boil, stirring constantly and simmer for 1 minute until thickened. Set aside.
Preheat the oven to 110°C/gas mark ¼. Pulse the biscuits in a food processor to crush them. Combine with the melted butter and a pinch of salt. Press firmly and evenly across the base and partway up the sides of a 23cm springform cake tin. Chill for 15 minutes.
Whisk the cream cheese in a large mixing bowl until smooth. Using a wooden spoon, beat in the sour cream, cornflour, sugar, lemon zest and vanilla. Beat in the eggs, one by one, stopping when the mixture is just smooth. Pour half the mixture into the tin and dot teaspoons of blackberry purée over the surface.
Cover with the remaining cream cheese mixture, level the top and drag teaspoonfuls of the remaining blackberry purée over the surface, running a skewer or spoon handle through to create a marbled pattern.
Place the tin on a baking sheet, slide on to the middle shelf and bake for about 1¾ hours, until set, but still slightly wobbly in the middle. Turn off the oven, prop the door ajar and leave the cheesecake to cool completely. Chill for at least 4 hours or overnight. Run a knife around the edge and remove from the tin before slicing.