Heat the coconut milk, the Natvia, remaining vanilla, remaining water and a pinch of salt in a saucepan until almost boiling. Remove from the heat and stir through the gelatine mix. Whisk with an electric mixer for 5-10 minutes until smooth and thick. Spread the 2 tbsp jam (if using) over the biscuit base, and then pour the marshmallow mix over the biscuit base and return to freezer for another 20 minutes or until the marshmallow has set.