Make the base. Place the biscuits in a food processor or blender and whizz until small crumbs form. Alternatively, place the biscuits in a plastic food bag, seal and crush with a rolling pin. Mix together the biscuit crumbs, melted butter and sugar. Press the crumbs into the base of a lightly greased 20 cm (8 inch) spring-form cake tin. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 10 minutes until crisp.
Meanwhile, beat the cream cheese in a bowl for 1 minute until smooth, then add the sugar, flour, vanilla extract, eggs and egg yolk and half the soured cream and beat together until smooth. Spoon the mixture into the tin, return to the oven and bake for a further 40 minutes until just set. Leave to cool in the tin, then chill overnight.
When ready to serve, stir the remaining cream until smooth. Remove the cheesecake from the tin and place on a serving plate, then spoon over the cream. Halve the passion fruits and spoon the seeds and pulp over the cake.