To make the tart dough, combine the flour, cocoa, salt and sugar in the bowl of a food processor and pulse a few times to combine. Add the butter and pulse until the butter resembles the size of peas. Stir the egg yolk to break it up and add a little bit of the egg yolk and pulse to combine. Repeat until all of the egg yolk is added. Then process the dough in 10 second bursts until the dough forms clumps. Turn out the dough and knead lightly to incorporate the remaining dry ingredients, where necessary.
Press the dough into a 23cm (9 inch) deep loose bottomed tart tin or 8 mini tart tins. Freeze the tart bases for 30 minutes to 1 hour before baking. When ready to bake preheat the oven to 190 degrees celsius (375 fahrenheit) and line with foil and pie weights and bake for 15 minutes, remove the weights and bake for a further 5 minutes. Set aside to cool while you prepare the filling.
To make the salted caramel place the sugar and water in a medium sized saucepan over a low heat. Stir until the sugar dissolves and then add the butter. Stir until the butter melts and then increase the heat to medium and allow the mixture to cook until it reaches a light toffee colour, around 15 minutes. Add the cream and sea salt, stir to combine and then cook for another 2 to 3 minutes or until the sauce has thickened slightly. Set aside to cool.
Preheat the oven to 180 degrees celsius (350 fahrenheit).
For the chocolate layer, gently melt the chocolate and butter in a bowl over a pan of simmering water. Once melted mix to combine and set aside while you prepare the remainder of the filling. Combine the eggs, egg yolks and sugar in the bowl of an electric mixer fitter with a paddle attachment and whisk until light and creamy. Add a small amount of the chocolate to the mixture and whisk to combine, to temper the eggs and prevent them from curdling, before adding the remaining chocolate. Whisk until smooth.
To assemble the tarts pour the caramel into the base of the tart tin, or divide evenly between the mini tart tins if using, and then top with the chocolate mixture. Place on a baking tray (to stop any overflowing caramel making a mess in your oven) and bake for 20 minutes for the large tart and 10 to 12 minutes for mini tarts. The tart should almost be set with the center still a little wobbly. Allow to cool in the tin before removing to serve.