To make the pastry, sift the flour, icing sugar and cocoa together into a bowl. Add the butter and rub it in with your fingers until the mixture resembles breadcrumbs. Add the egg yolks and 3 tbsp ice-cold water, and continue to mix with your fingers until you have a soft dough. Alternatively, do this in a food processor by pulsing the ingredients together. Wrap the dough and chill for at least 1 hr. Will keep chilled for up to two days.
Roll the dough out on a lightly floured surface to the thickness of a £1 coin, then use it to line a 23cm loose-bottomed tart tin, working the pastry to the edge and leaving some overhanging. Cover and put in the fridge to chill for a further hour.
Heat the oven to 190C/170C fan/gas 5. Line the case with baking parchment, then fill with baking beans. Bake for 20 mins. Remove the baking beans and parchment, and bake for a further 15-20 mins, or until the base is crisp. Leave to cool completely. Once cooled, use a sharp knife to trim off the excess pastry and create a smooth edge.
For the filling, melt the butter and dark chocolate together in a heatproof bowl set over a pan of simmering water, making sure the bowl doesn’t touch the water. Stir until smooth, remove from the heat and leave to cool slightly. Beat the eggs and sugar with an electric whisk in another bowl for 5-6 mins, or until thick and pale. Fold in the chocolate, then sift in the cocoa.
Scatter the caramel chocolates over the base of the pastry case, then pour over the filling. Transfer the tart to a baking sheet and put in the oven (still set at 190C/170C fan/gas 5) for 20-25 mins, or until set and the top has formed a crust. Leave to cool to room temperature, then carefully remove from the tin and put on a serving plate. Dust with the extra cocoa and sprinkle over a little sea salt, if you like.