Preheat oven to 160 C / Gas 3. Fill a roasting tin 1/2 full with water and place in the oven to heat up. Line an 18cm springform tin with baking paper.
Mix biscuit crumbs with sugar in a large bowl; stir in melted butter and mix well to combine. Spoon the mixture into the bottom of the tin, spreading it out evenly and pressing it down with the back of a spoon to compact the base. Place in the fridge while preparing the filling.
Beat eggs and caster sugar together in a large bow until light and fluffy. Stir in ricotta and cream cheese until well combined. Add lemon extract, lemon juice, lemon zest and cornflour; stir well to thicken slightly.
Spoon lemon-ricotta cheesecake filling on top of chilled biscuit base, using a spatula to level out the top.
Bake cheesecake in water bath for 40 minutes or until it appears firm on the surface, but still has a wiggle. If the cheesecake is browning too much - cover it with foil until the end of cooking. The steam in the oven will help ensure the cheesecake does not dry out.
Turn off heat and remove the cheesecake from the water bath but let the cheesecake cool in the oven for an hour or two. Cover cheesecake and store in the fridge for at least 6 hours or overnight.
When ready to serve; warm lemon curd in a pan over low heat until runny. Spoon the warm lemon curd on top of the cheesecake and serve.