Zest the lemons and limes and set the zest aside
Juice the lemons and limes through a sieve and weigh the liquid
Add water so the total weight of liquid is 260g
Add 125g sugar and stir in a pan over high heat until it boils, then remove from heat
Whisk together mascarpone, ricotta, icing sugar and citrus zest until fairly smooth
Add half of the cream and whisk until just combined
Add the other half of cream and whisk for a couple of minutes until it thickens to a soft spreadable texture
Cover the bottom of serving dish with a thin layer of the cream
Soak half the sponge fingers in half of the syrup, flipping after about 10 seconds
Lay the soaked sponge fingers over the dish and cover with half of the lemon curd and half of the cream mixture
Soak the second half of sponge fingers in remaining syrup and repeat the layering
Finish with remaining curd and a top layer of remaining cream
Chill for at least 5 hours or overnight
