Line a 20cm square brownie tin with baking paper. Tip all of the ingredients, except the salted caramel sauce and salt, into a large pan over a low heat and stir until the sugar has melted. Turn up the heat and simmer for 15-20 minutes, stirring regularly to avoid the mixture catching, until it reaches 115C on a sugar thermometer. If you don’t have a thermometer, carefully drop ½ tsp of the mixture into a glass of cold water. Leave for 30 seconds to cool, then pick up the ball of syrup and squeeze between your fingers – it should be slightly firm but malleable. If it’s too syrupy and dissipates in the water then the fudge needs longer on the hob. Repeat this process after another 2-3 minutes.