Preheat the oven to 160°C/325°F/gas 3. Crush the biscuits in a food-processor, then mix with the butter. Press onto the base and halfway up the sides of the prepared tin. Bake for 5 minutes, then cool.

Using an electric mixer on low speed, beat the cream cheese with the flour, sugar, vanilla, eggs, egg yolk and half the soured cream until thoroughly combined. Don’t let the mixture become frothy as this will spoil the creamy, even texture of the cheesecake. Stir in the raisins, reserving a few for decoration, if you like.

Pour the mixture into the tin on top of the biscuit base. Set the tin on the baking sheet and bake for 40 minutes. Stir the remaining soured cream, then carefully pour it over the top of the cheesecake. The top will still be fragile so if you need to spread the soured cream, do this gently. Bake for a further 15 minutes or until the cheesecake is set but still slightly wobbly in the centre.

Remove from the oven and run a round-bladed knife around the inside of the tin to loosen the cheesecake. Leave to cool in the tin, then cover and chill overnight before unmoulding for serving and scattering with the reserved raisins, if you like.