Line a 20cm square baking tin with greaseproof paper.
Place all the ingredients, apart from the salt, into a large nonstick pan. Bring to a rolling boil at 112–114°C for 10 minutes, stirring constantly.
Take off the heat, cool for 2 minutes, then whisk vigorously by hand for a minute before pouring into the lined tin.
Sprinkle over the sea salt and leave to cool at room temperature for 2 hours.