Prepare the Crust: Preheat your oven to 180°C (160°C for fan ovens) / 350°F / Gas Mark 4. Lightly butter a 23 cm (9-inch) springform pan to prepare it for baking. Mix the crushed digestive biscuits with melted butter and sugar in a bowl until combined. Press the mixture evenly into the base of the prepared tin. Bake the crust for 10 minutes, then remove from the oven and let it cool.
Make the Cheesecake Filling: Beat the cream cheese, sugar, and flour in a large mixing bowl using an electric mixer at medium speed until smooth. Add the vanilla extract, lemon zest, and lemon juice. Mix well. Add the eggs and yolk, one at a time, mixing on low speed until just incorporated. Avoid overmixing to prevent air bubbles. Fold in the soured cream gently until smooth and creamy.
Bake the Cheesecake : Pour the filling over the cooled crust and smooth the top with a spatula. Set the cheesecake pan on a baking sheet. Bake for 50–60 minutes, or until the edges are firm and the center has a gentle wobble. Once the baking time is up, turn off the oven and let the cheesecake cool slowly inside with the door slightly cracked open for about an hour.
Prepare the Soured Cream Topping: Mix the soured cream, sugar, and lemon juice in a bowl until smooth. Spread the mixture evenly over the cooled cheesecake.
Chill and Serve Allow the cheesecake to chill in the refrigerator for a minimum of 4 hours, or preferably overnight, for optimal results. Run a knife around the edge of the tin to loosen the cheesecake before removing the sides. Slice and serve chilled.