Grease and line a 23cm round tin with a removable base.
Bash the biscuits up in a food processor, or in a sandwich bag using a rolling pin. Pour into a bowl and stir in the melted butter. Keep stirring until this is all combined and then spoon into the lined tin and flattened with the back of your spoon so the whole base is equally covered and put in the fridge.
In a large bowl mix the cream cheese with the lemon zest and the vanilla seeds and caster sugar. Cut the vanilla pod in half lengthways and use your finger to remove the seeds. Mix the cream cheese until you can see an equal distribution of vanilla seeds.
Spoon on top of the biscuit base and level off. Put in the fridge to firm up for one hour. Remove from the tin and put on a plate to serve, let it come up to room temperature for 30 minutes before serving.