Whisk the eggs and castor sugar together on high speed for 2 minutes. Whisk in the vanilla extract. Set aside.
Heat the milk in a large saucepan until it is just about to reach boiling point. Remove from heat. Whisk about 1/2 a cup of the hot milk into the egg mixture. Pour the egg mixture back into the saucepan with the remaining milk.
Cook the egg custard mixture over a very low heat for 3 minutes while whisking continuously. Remove from the heat and whisk in the cream. Set aside to cool completely.
Prepare the caramel by dissolving the castor sugar in the water over a low heat. Once the syrup begins to bubble you can stop stirring it. Use a pastry brush dipped in water to wash away any sugar crystals that collect around the sides.
Allow the sugar syrup to boil gently for 12-15 minutes or until it turns a deep golden colour. Keep a close eye on the pot as sugar syrup can burn quickly! Remove from the heat and add the cream, salt flakes and butter. Whisk vigorously but carefully as the mixture will bubble up and steam. Continue to whisk until the sauce is smooth. Set aside to cool at room temperature.
Once the egg custard mixture is cool whisk in half a cup of cooled caramel sauce. Pour the mixture into your ice cream maker and churn as per the instructions. Towards the end of churning stir the cream caramel toffees into the ice cream.
Store in the freezer. Serve with lashings of leftover caramel sauce.