Start by making the filling. Whip together the double cream and icing sugar to soft peaks. Add in the 1 tbsp lemon curd, mascarpone cream, lemon zest and limoncello liqueur and continue to whip till incorporated. Next, place the soaking syrup in a bowl with the 2 tbsp limoncello and mix together well. Start by soaking the savoiardi biscuits in the syrup for 5 seconds and lay side by side in the base of a square dish. Spoon over half of the mascarpone filling and even out. Mix the remaining lemon curd in a bowl with a little water to make it runny and drizzle half over the filling. Repeat this process again with the fingers, filling and lemon curd. Place into the fridge to chill for 4 hours or overnight. Before serving, decorate the top with extra lemon zest.