Pour 3 tbsp of water into a bowl and sprinkle with gelatin. Set aside.
In a saucepan over medium heat, combine the sugar and ¼ cup water and bring to boil. Stir until the sugar has dissolved. Boil for about 5 minutes without stirring until the syrup is an amber colour.
Add the butter, ½ cup cream, vanilla, gelatin and fleur de sel (watch out for splashes). Remove the pan from the heat and stir well for 2 minutes.
Let it cool and reserve ½ cup for toppings.
In a bowl using an electric mixer, whip the rest of the cream until it forms stiff peaks. Gently fold the caramel into the whipped cream using a spatula.
Divide the mousse into the glasses and leave to set in the refrigerator at least 2 hours or more.
When ready to serve, garnish with a caramel coulis, nim-nums, pieces of caramel and caramel popcorn.