Heat the oven to 200C/180C fan/ gas 6. Butter the side of a 23cm springform tin and line with baking parchment. Put the biscuits in a food processor and blitz to a fine crumb, then add the butter and blitz again to combine. Tip into the tin, then press down firmly into an even layer and bake for 10 mins. Set aside.

Tip 25g of the sugar and the blackcurrants into a small pan. Mix the cornflour with 1 tbsp cold water, then add to the pan. Warm over a low-medium heat for 1-2 mins until the sugar has dissolved, then turn up the heat to medium and bubble for 1-2 mins until saucy. Set aside.

Whisk the soft cheese in a large bowl using an electric whisk until smooth. Add the remaining sugar and whisk again, then add the flour, vanilla, eggs and soured cream, then whisk for a third time until smooth and well combined.

Pour half the cheesecake mixture over the cooled biscuit base, then dot over half of the cooled blackcurrant sauce. Pour over the remaining cheesecake mixture, and carefully smooth the surface using a spatula. Drizzle over the remaining blackcurrant sauce, then gently ripple it through the cheesecake mixture using a skewer or the end of a spoon. Bake for 10 mins, then reduce the oven temperature to 110C/90C fan/gas 1/4 and bake for a further 30 mins until set with a slight wobble in the middle. Turn off the oven and leave the cheesecake inside with the door closed for 1 hr, then open the door slightly and leave for 1 hr more. Finally, leave to cool completely at room temperature, then chill for at least 3 hrs or overnight before slicing. Will keep chilled for five days.