In a large mixing bowl, whip the double cream with the icing sugar until soft peaks form.
Add the mascarpone cheese, lemon zest, lemon juice, and vanilla extract to the whipped cream. Continue to whisk until the mixture is smooth and thickened.
Lay a layer of digestive biscuits in the base of a rectangular baking dish, ensuring they are slightly overlapping.
Spread half of the lemon mixture evenly over the biscuits.
Repeat the layers, starting with digestive biscuits and finishing with the lemon mixture on top.
Cover the dish with cling film and refrigerate for at least 4 hours, or preferably overnight, to allow the flavours to meld and the cake to set.
Once ready to serve, slice into squares and enjoy.
