Crush the biscuits in a grinder (or break them up by hand). Mix with the butter and extra-virgin olive oil to make a smooth paste. Flatten it out with a rolling pin to form the base in a cake mould. Press it down well to give a firm base.
To make the cheesecake mousse, mix all the ingredients together in a blender to make a thick, creamy mixture.
Make the lemon layer by bringing the grated lemon zest, sugar and water to the boil. Leave to steep for five minutes and then strain. Reheat, add the hydrated gelatine and leave to dissolve.
Cover the biscuit base with the cheesecake mousse and then add the lemon layer while still warm.
Put an airtight lid on the cake mould and cool in the fridge for at least six hours before serving.