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Preheat the oven to 190°C (375°F/Gas 5). Line the base of the tin with baking parchment.
To make the cheesecake base, mix the butter, sugar, salt, vanilla seeds, and flour in a bowl and use your fingertips to rub everything together into a rough crumble. Spread the crumble over the base of the tin and press down well, making sure that all areas are covered. Bake on the middle shelf of the preheated oven for around 10 minutes until golden brown. Remove, brush with the egg yolk, and bake for another 5 minutes. Remove from the oven and leave to cool in the tin. Meanwhile, reduce the oven temperature to 150°C (300°F/Gas 2).
For the filling, stir together the cream cheese with the soured cream, sugar, eggs, vanilla seeds, lemon zest, and salt in a high-sided bowl, using a hand-held blender to produce a smooth cream. Take care not to incorporate any air in the mix.
Spoon the filling onto the cooled base. Bake on the middle shelf of the oven for 30–35 minutes. The mix should still wobble slightly in the centre, like a jelly; it will become firmer as it cools down.
Let the cheesecake cool in the tin on a wire rack for at least 2 hours. Then cover and refrigerate while you prepare the passion fruit jelly.
To make the jelly, soak the gelatine in a bowl of cold water for 10 minutes. Heat the passion fruit purée in a saucepan over a low heat, squeeze out the gelatine and let it dissolve in the warm purée (don't let it get too hot). Carefully spread the passion fruit jelly over the chilled cheesecake. Cover and refrigerate once more for around 2 hours, so that the jelly can set. Keep it there until you are ready to serve. Decorate the edge of the cheesecake with white chocolate shards or toasted coconut flakes (if using).