Melt 200g of the sugar in a pan over a medium heat until golden, then beat in 400g more, along with the double cream, butter and 1 teaspoon of sea salt.
Cook, stirring, over a medium-high heat, until it reaches 118ºC on a sugar thermometer (always be careful when boiling sugar).
Off the heat, beat with an electric mixer until thick. Pour into a lined 20cm x 20cm tin and spread out evenly with a spatula.
Leave to cool, then chill in the fridge for 2 to 3 hours to set. Sprinkle with a pinch of sea salt, cut into 2 to 3cm squares and serve.