For the pastry, pour boiling water into a small saucepan to a third full and sit small bowl on top. Add chocolate and allow to stand, until chocolate melts, stirring occasionally.
Meanwhile, place the cocoa, flour, sugar and butter together in a food processor and pulse until mixture resembles fine breadcrumbs. In a small bowl combine egg yolk, milk and vanilla, whisking until combined. Using a spatula, add to flour mix and pulse until combined. While the food processor is running, slowly add the melted chocolate until the mixture starts coming together as a ball. Remove dough onto a lightly floured surface and bring together as a smooth ball. Flatten into a disc, cover with plastic wrap and refrigerate for 2 hours.
Preheat oven to 180°C (160°C fan-forced), lightly grease a 12cm x 36cm rectangular tart tin with removable base and place on a baking tray lined with baking paper.
Place pastry on a lightly floured kitchen work surface; roll pastry to a thickness of approximately 3mm. To check if the pastry is the correct size for the tart tin, place the tart tin over the pastry; there should be at least a 5cm border the whole way around.
Carefully line the tart tin with pastry, prick the base all over with a fork, cover and chill in the refrigerator for 30 minutes.
To blind bake the tart shell, line the pastry case with baking paper or foil. Cover paper with baking beans or a dried pulse to help support the sides and weigh the base down during cooking.
Place tart tin on a baking tray in preheated oven and cook for 15 minutes. Remove from oven, check pastry edges are supporting themselves before removing beans and baking paper. Return tart tin to oven and cook for a further 15 minutes then allow to cool completely.
To make the salted caramel, place sugar and ⅓ cup water in a medium saucepan over a low heat, stirring until sugar dissolves. Increase the heat to medium-high, bring to the boil and cook sugar syrup for 10 minutes or until mixture turns a light golden colour. Remove from heat, add cream, stirring until well combined (be careful as it may spatter). Add butter a cube at a time, stirring until melted and combined. Add the salt flakes, vanilla and macadamia nuts stirring until combined. Pour salted caramel into the cooked tart shell and smooth. Refrigerate for 2 hours or until set.
To make the ganache, place prepared chocolate in a medium mixing bowl.Place cream in a small saucepan over a medium heat, bring to just below boiling point. Pour over chocolate and gently stir to combine. Leave to stand for several minutes before stirring again. Repeat process until chocolate has melted, making sure mixture is not stirred too much, then allow to cool for 15 minutes.
Carefully spread the ganache over the top and leave for 2 hours or until set.