Preheat the oven to 180°C / 350°F / Gas mark 4.
Line the base of a 23cm springform cake tin with parchment paper and set aside.
Melt the butter in a medium pan. Stir in the biscuit crumbs and sugar so the mixture is moist evenly.
Press the mixture into the bottom of the pan and bake for 10 minutes.
Cool on a wire rack.
Increase the oven temperature to 200°C / 400°F / Gas mark 6.
Beat the soft cheese until creamy, mixer or food processor ideally.
Gradually add the sugar, then the flour and a pinch of salt, scraping down the sides of the bowl.
Whisk in the vanilla, lemon zest and juice.
Whisk in the eggs and yolk, one at a time, scraping the bowl.
Stir the 284ml of soured cream until smooth, then measure 200ml on low speed as you add the measured soured cream (reserve the remainder).
Whisk to blend and make smooth and light.
Brush the sides of the spring form tin with melted butter and line with a baking sheet.
Pour and scrape in the filling making sure it is as smooth as possible.
Bake for 10 minutes.
Reduce oven temperature to fan 90°C /Gas ¼ and bake for 25 minutes more. If you gently shake the tin, the filling should have a slight wobble.
Turn off the oven and open the oven door for a cheesecake that's creamy in the centre. Let cool in the oven for 2 hours.
Combine the reserved soured cream with the 142ml carton, the sugar and lemon juice for the topping.
Spread over the cheesecake to the edges.
Cover loosely with foil and refrigerate overnight.
Run a round-bladed knife around the sides of the tin to loosen any stuck edges.
Unlock and slide the cheesecake off the bottom of the tin onto a plate, then slide the parchment paper out from underneath.