In a small saucepan over medium-low heat, heat the cream and butter until the butter has melted. Stir in the vanilla extract and set aside.
In a large saucepan over low heat, stir the sugar with the boiling water until the sugar has dissolved. Use a pastry brush dipped in cold water to brush the sugar crystals from the side of the saucepan. Stop stirring, increase the heat to medium and simmer until the caramel reaches a light amber colour. Remove from the heat and add the cream/butter mix carefully (as it will boil), whisking constantly until smooth. Add the salt to taste and stir well.
Immediately pour into sterilised jars and screw the lids on firmly. Store unopened at room temperature for up to 3 months. Refrigerate once open and use within a week. Reheat portions in the microwave on medium power.
Makes 2 small bottles.