To make the pastry, process flour, cocoa, icing sugar and butter in a food processor until the mixture resembles fine breadcrumbs. Add the egg yolk and water and pulse until just combined. Press the dough together into a ball, flatten into a disk, cover with cling wrap and refrigerate for 15 minutes.
Divide dough into 10-12 even portions. Roll out each portion until 3mm thick. Line a 2cm deep 7 cm fluted flan tin, with removable base, with the pastry and trim excess. Repeat with remaining pastry, re-rolling any excess pastry, if necessary. Place tins on a baking tray and chill in freezer for 30 minutes.
Cover each pastry shell with baking paper and fill with pastry weights. Bake at 200°C for 10 minutes. Remove weights and paper and bake for a further 5 minutes or until cooked through and try to the touch. Cool completely.
To make the caramel, combine all ingredients in a saucepan over medium-low heat. Cook, stirring for 8-10 minutes or until golden. Spoon caramel evenly over each pastry case. Cool.
To make the chocolate ganache, place chocolate and cream in a heat-proof bowl over a saucepan of simmering water. Stir until melted. Pour over the caramel. Refrigerate to set.
Remove tarts from fridge 30 minutes before serving, and sprinkle with extra sea salt.
If sea salt is unavailable, just add table salt to the caramel only. Pastry can be made ahead and frozen, for up to a month. Pastry tart cases can be cooked ahead of time and refrigerated overnight.