Put the sugar, 1 tsp of fine sea salt and 100ml of water in a large, heavy-bottomed pan over a high heat. Cook for 10-12 minutes without stirring, swirling the pan occasionally, until the sugar is smoking, nutty smelling and a very dark caramel colour – the sugar will become foamy and lava-like with slow-bursting bubbles. Reduce the heat to low and immediately add the cream-milk mixture in a thin, steady stream, stirring with a whisk as you do – this stops the cooking process and prevents the sugar from burning, and will cause the sugar to harden. Increase the heat to high and cook for 5-7 minutes or until the seized sugar has dissolved and the mixture is liquid again. Turn off the heat.