In a small saucepan over medium heat combine lemon juice, lemon zest, sugar and egg yolks. Stir constantly with a wooden spoon. The mixture should start simmering and thickening. When it thickens to the point of the spoon remaining coated when raised, remove the pan from heat.
Let cool until room temperature, whisking from time to time. You can leave the lemon curd in the fridge and make the cake on the next day.
In a large bowl beat together lemon curd and mascarpone until well combined.
In a separate bowl whip the heavy cream until thick. Using a rubber spatula carefully fold the cream into the mascarpone and lemon mixture, until the mousse becomes uniform.
Arrange a layer of cookies in a rectangular cake pan (alternatively you can use a round pan and break the cookies in small pieces before spreading them). Cover generously and evenly with lemon mousse. Continue the process until using all biscuits and all the lemon mousse.
Wrap the pan with cling film and transfer it to the fridge. Refrigerate for at least 6-8 hours.
Before serving decorate with chocolate sprinkles.