Allow the cake and curd layers to cool before adding the meringue.
Preheat oven to 200°C. Line and grease a 22cm springform tin with cooking spray. Blitz the base ingredients in a food processor until well combined. Press into the base of the tin.
Bake for 10 to 12 minutes or until golden. Leave to cool.
Filling: whisk the cream cheese, sugar, sour cream, cornflour, lemon zest, vanilla, and salt with electric beaters on medium speed until smooth. With the mixer on low speed, beat in eggs until fully combined. Pour on to the cooked base.
Pour enough boiling water into a deep oven tray to cover the base and place on the lower rack in the oven. Bake the cheesecake on the upper rack for about 1 hour, until set and pale golden.
Turn off the oven, leave the door open and let the cheesecake cool for about 1 hour.
Lemon curd: pour water into a medium pan to a depth of 3 - 4 cm. Bring to a simmer over medium-high heat. Combine egg yolks, sugar, lemon zest, lemon juice, cornflour and salt in a heatproof bowl and place on top of the pan. Whisk the mixture over the simmering water until thick. This will take about 10 minutes.
Remove the bowl from the pan. Whisk in the butter, a piece at a time until melted and fully combined.
Strain the curd through a fine-mesh sieve into a medium bowl.
Pour the curd over the cheesecake and smooth in an even layer. Refrigerate, uncovered, for at least 3 hours or up to 24 hours.
Meringue: whisk egg whites and sugar together over a bain-marie of boiling water until the mixture is about 40°C, warm to touch and the sugar has just melted. This is called French meringue.
Transfer the egg white mixture to an electric mixer and whizz until stiff. Add a pinch of salt.
Place the cheesecake on a large baking sheet and gently remove the ring of the tin, keeping the cheesecake on the base. Spoon meringue on top of the lemon curd. Spread to the edge of cheesecake and make swirls with a spoon.
Place an oven rack about 4cm from the grill and set to the highest heat setting. Grill the meringue for about 1 - 2 minutes, watching carefully and turning as needed, until the meringue is lightly and evenly browned. Or use a kitchen blow torch to toast the meringue.