1. Pierce the tops of the cans of condensed milk once and place cans in a saucepan. Add boiling water to just below the top level of the cans.
2. Place over medium-low heat and simmer gently for three hours,topping up with water as needed to keep the cans almost covered. Remove and set aside to cool.
3. Process the biscuits in a food processor until they have the consistency of fine crumbs. Pour in the melted butter and pulse to combine. Evenly press mixture into the base of a 1.5-litre pie tin. Chill in the refrigerator until needed.
4. Place the gelatine in 2 tbsp of cold water for five minutes,then pour into a small saucepan with half the whisky and heat gently until gelatine is dissolved. Allow to cool slightly. Whisk in the remaining whisky.
5. Empty the cans of caramel into a large bowl,season with the salt,then whisk in the whisky mixture until smooth. In a separate bowl,whisk the cream and vanilla to firm peaks,reserving ¼ cup to serve. Gently fold half of the cream into the caramel mixture,then the remaining half,until combined.
6. Pour the mixture into the base and chill in the fridge for at least 4 hours.
7. Serve with the remaining cream spooned on top.