Add the butter to a saucepan and melt over a low heat. Blitz the biscuits to a coarse crumb in a food processor. Add the sugar and combine. Once the butter has melted, slowly pour it into the food processor, mixing on a low speed as you do so. Stop when just combined.
Transfer the mixture to a large cake tin and flatten to create an even base. Press down firmly and chill in the fridge while you prepare the rest of the cheesecake.
Place the cream cheese in a food processor and beat at a medium-low speed with the paddle attachment until smooth and creamy. Slowly add in the sugar, then the flour and a pinch of salt. Scrape the mixture from the sides of the mixer a couple of times to ensure it is well mixed.
Change to the whisk attachment and then whisk in the vanilla and the zest and juice of the lemon. One by one, whisk in the eggs and the yolk, scraping down the side of the mixer a couple of times again. Combine well and then mix in the soured cream.
Spoon the mixture on top of the biscuit base and flatten the surface to create an even layer.
Mix together the soured cream, sugar and the lemon juice. Spread this mixture over the cheesecake to give an even topping. Cover the cheesecake loosely with foil or clingfilm and chill in the fridge for at least 4 hours.
When removing from the tin, run a knife around the edge or gently warm the sides with a blowtorch to make it easier.
«Take care not to over-beat the mixture - you just want to combine each addition to give a smooth, light and somewhat airy cheesecake.»