Preheat the oven to 180.C (fan 160.C)/350.F/gas mark 4. Grease the base and side of a 23cm (9in) springform cake tin with extra butter, and line the base with baking paper.
For the base, melt the butter in a medium pan, then stir it into the biscuit crumbs. Press the mixture into the booto of the prepared tin and bake in the preheated oven for 10 minutes. Cool on a wire rack
While preparing the filling.
For the filling, increase the oven temperature to 220.C (fan 200.C)/ 425.F/gas mark 7.
In a mixer fitted with the paddle attachment, beat the cream cheese at medium-low speed until creamy, about 2 minutes, then gradually add the sugar, then the flour. Continue by adding the vanilla and lemon zest and juice, then whisk in the eggs, one at a time. Stir in the soured cream until well blended. The batter should be smooth, light and somewhat airy.
Pour the filling into the prepared tin. The top should be as smooth as possible. Bake in the preheated oven for 10 minutes. Reduce the oven temperature to 140.C (fan 120.C)/275.F/gas mark 1, and bake for 25 minutes more. If you gently shake the tin, the filling should have a slight wobble. Turn off the oven. Leave the cheesecake to cool in the oven for 2 hours, with the door closed. The cheesecake may get
A slight crack on top as it cools.
For the topping, combine the soured cream and caster sugar. Spread over the cold cheesecake right to the edges, then refrigerate overnight. Dust with icing sugar before serving.
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