Heat the oven to 180C/160C fan/gas 4 and put an ovenproof dish half-filled with water on the bottom shelf. For the base, combine the digestives and melted butter in a bowl, then tip into a deep 23cm springform cake tin. Press into the base and up the side using a spatula. Set aside on a baking tray.

Whisk the cheese, sugar, cornflour, vanilla and soured cream together, then add the eggs, one at a time, beating until creamy. Mix in the chopped pistachios, then pour the filling over the base. Bake on the middle shelf for 10 mins, then reduce the oven to 150C/130C fan/gas 2 and bake for 40 mins more until set with a slight wobble in the middle. Turn off the oven and leave the cheesecake inside with the door ajar for 30 mins (to avoid cracks). Leave to cool completely, then chill for 24 hrs. Pour over the pistachio cream and sprinkle with the ground pistachios to serve. Will keep chilled for up to two days.