Place the castor sugar and water in a medium sized saucepan and stir over a low heat to dissolve the sugar. Use a pastry brush dipped in hot water to brush away any sugar crystals that collect on the sides of the pot.
Allow the syrup to boil for 10 minutes without stirring until the syrup is a deep golden colour. Keep a close eye on the pot as sugar syrup burns very quickly!
Take the saucepan off the heat and carefully add the cream while whisking. (Take care as the sauce will bubble up.) Add the salt flakes and whisk until smooth. Lastly add the butter and whisk until you have a smooth, glossy sauce.
Place the dark chocolate in a mixing bowl. Pour half a cup of hot caramel sauce over the chocolate and allow to stand for a minute. Whisk well until all the chocolate has melted and the mixture is smooth. (The remaining sauce can be poured into a glass jar or bowl and stored in the fridge.)
Pop the bowl in the fridge for at least 1-2 hours until the mixture is firm enough to roll. It can also be left overnight (covered).
Roll teaspoonfuls of the mixture into balls. Dust generously with cocoa powder and store in the fridge.