Preheat oven to 350°F (180° C).

Place 10 tbsp butter and sugar in a food processor until pale in colour and fluffy. Add cocoa, egg yolks, and vanilla, process until a smooth dough forms.

Press dough into 26 cm loose-bottomed fluted tart tin (to about 5 mm thick on bottom and sides). Refrigerate until firm, about 15 minutes.

Bake for 15 minutes or until cooked. Set aside.

To make caramel, place whipping cream and 6 tbsp of butter in a medium sauce pan until melted. Set aside. Place sugar and water in a large sauce pan over low heat, stirring until sugar has dissolved. Turn heat to high and bring to a boil without stirring until dark golden brown (340°F/170°C if you have a candy thermometer, if not this took me exactly 13 minutes from when the mixture started to boil). Remove from heat and CAREFULLY whisk in the butter/cream mixture (it will bubble up and splatter). Return the pan to low heat and stir until mixture has thickened, about 5 minutes. Pour into pastry shell and refrigerate until firm.

To make the chocolate glaze, heat the 1/2 cup whipping cream over low heat. Add chocolate and stir until melted. Pour over caramel layer and refrigerate until firm.

Sprinkle with sea salt flakes and cut into wedges to serve.