Put the cocoa powder, icing sugar and flour into a food processor with the cubed butter and pulse a few times until the mixture combines to resemble breadcrumbs.
Add the egg and water and process until the mixture clumps into a dough.
Alternatively, place in a large bowl and use a fork, pastry cutter or cold hands to rub the butter into the flour.
Gradually mix in the egg and water with a dinner knife until well combined and bring the mixture together to a dough.
Knead very lightly and flatten into a disc.
Cover in plastic wrap and chill for 30 minutes.
Preheat the oven to 190 C / 375 F / 170 FAN / Gas 5
Roll out the pastry to about 0.5 centimetres thick to line the tin between 2 pieces of plastic wrap and prick with a fork
Line the pastry with baking parchment and fill with baking beans.
Bake for 12 minutes then check that the pastry has set.
Remove the beans and paper carefully and allow the pastry to cool.
While the pastry is baking place the butter and chocolate into a heatproof bowl over a saucepan filled with simmering water.
Stir occasionally until melted and combined then remove from the heat.
In another bowl beat together the egg, egg yolk and sugar then beat in the chocolate mix.
Spread the caramel over the base of the tart with the spatula then rinse and use it to smooth the chocolate layer over the top.
Bake for a further 10 minutes.
Cool and serve chilled.