Bring the sugar, water, lemon juice and salt to the boil in a wide pan without stirring. Reduce the heat and simmer, swirling the pan occasionally until a light brown caramel has formed. Remove the pan from the heat immediately. Add the chestnuts, swirl briefly in the caramelized sugar, pour in the stock. Cover and simmer over a low heat for approx. 15 mins. until the sugar has dissolved, stir in the butter.