To make the salted caramel, Place the cream and butter in a small saucepan over medium heat and bring to the boil. Remove from the heat and set aside.
Place the sugar and water in a medium saucepan over low heat and cook, stirring, until the sugar is dissolved. Place a sugar thermometer in the pan increase the heat to high. Bring to the boil and cook for 10–12 minutes, without stirring, or until the temperature reaches 150°C and the mixture is a deep caramel colour. Remove from the heat and working quickly, add the salt and cream and butter mixture and whisk to combine. Return to the heat and cook for a further 2 minutes or until thickened slightly.
Place the cocoa, flour, butter and icing sugar in a food processor and process until the mixture resembles fine breadcrumbs. With the motor running, add the egg yolk. Add the iced water and process until the dough just comes together. Turn out onto a lightly floured surface and gently bring together to form a ball. Flatten into a disc, wrap in plastic wrap and refrigerate for 1 hour.
Preheat oven to 180°C. Roll the pastry out between 2 sheets of non-stick baking paper to 3mm-thick. Line a lightly greased 24cm round loose-bottomed tart tin with the pastry. Trim the edges and prick the base with a fork. Refrigerate for 30 minutes.
Line the pastry case with non-stick baking paper, fill with baking weights and bake for 15 minutes. Remove the paper and weights and bake for a further 10 minutes or until the pastry is just cooked. Allow to cool in the tin. Spoon the salted caramel into the tart shell and refrigerate for a 2–3 hours or until set.
To make the dark chocolate ganache, place the chocolate and cream in a small saucepan over low heat and cook, stirring, until melted and smooth. Allow to stand for 10 minutes or until thickened slightly. Pour the chocolate mixture over the caramel and refrigerate for 1–2 hours or until set. Bring to room temperature and sprinkle with sea salt flakes to serve. Serves 6–8.