1. Place the sugar in a heavy-based pan with 50ml water and heat, stirring occasionally, until the sugar has dissolved then heat gently to a rich, golden caramel colour.
2. Pour the cream into another saucepan and warm gently with the butter.
3. Remove the cream and butter from the heat and pour into the caramel. It will hiss and spit. Once calm and smooth, tip into a metal bowl, add the salt and leave until cool but not cold. Take 2 tbsp of the caramel and place in a piping bag to use later. Pour the rest into the pastry case and chill until set.
4. For the topping, heat the cream gently until just reaching a simmer and pour over the chocolate, then leave for a few minutes before stirring until the chocolate is smooth and has melted.
5. Pour onto the caramel and leave to cool and set in the fridge. Drizzle with reserved caramel and serve dusted with cocoa.