Mix together the cream cheese, lemon zest and vanilla until smooth. Stir in 100g sugar, then beat in the eggs one at a time until blended.
Pour the cheesecake mixture evenly over the two digestive biscuit bases. Shake gently to settle the filling, then bake in the preheated oven for 25 minutes. Remove the cheesecakes, but do not turn off the oven. Allow cheesecakes to cool before topping.
For the soured cream topping: Whisk together the soured cream, 100g sugar and vanilla until blended. Distribute the topping mixture over the two cooled cheesecakes. Bake cheesecakes in the oven at 180 C / Gas 4 for an additional 10 minutes. Cool completely, then refrigerate until serving.