Make the cake. Preheat oven to 170°C (150°C fan) mark 3. Lightly grease the base and sides of a 20.5cm (8in) round springform tin. Line the base of the tin with parchment.
Put the zest and 2tbsp juice in a mixing bowl with the butter, flour, eggs and 100g (31/2oz) caster sugar. Whisk together with an electric hand whisk briefly until the mixture is smooth and combined (don’t overwhisk). Tip the mixture into the tin and spread to level. Bake for 25-30min until the cake is golden, risen and a skewer inserted into the centre comes out clean.
Put the remaining lemon juice and remaining caster sugar in a small pan. Heat gently to dissolve the sugar, then bring to the boil and remove from the heat. Carefully poke holes into the top of the sponge with a cocktail stick or skewer and pour the hot syrup over the cake. Allow to cool completely in the tin.
Unclip and remove cake from tin. Clean tin. Turn base of tin over so it’s flat-side up. Take a 30.5cm (12in) sheet of baking parchment and lay it over the base of the tin. Carefully clip the paper-covered base back into the tin so paper is tightly stretched over base and any overhang is trapped between base and sides of tin, then line sides of tin with another piece of baking parchment. Peel and discard parchment from the bottom of the cake, then place into the prepared tin, flat side down.
Make the cheesecake. In a large bowl, beat the cream cheese, icing sugar, melted butter, lemon zest and half the juice together with an electric handwhisk until smooth.
In a separate bowl, whip the cream until it just holds its shape, then gently fold it into the cream cheese bowl until combined. Tip the cheesecake mixture into the tin and spread to make an even layer. Chill in the fridge for at least 6hr or overnight.
When ready to serve, beat the remaining lemon juice with the lemon curd. Unclip cheesecake from tin and slide on to a serving plate or cake stand. Peel off the parchment around the sides. Drizzle some of the lemon curd sauce over the cheesecake to decorate and sprinkle over the lemon zest and pearl sugar, if using. Serve in slices with a drizzle of extra sauce.
