With an electric beater combine flour, cold butter, cocoa, icing sugar and salt to make fine crumbs. Add the egg yolk and small amounts of ice water until it forms a kneadable dough. Form dough into a ball, wrap with plastic wrap and chill in the fridge 20 minutes.
Roll out the dough on a floured surface and place in the tart tin. Trim the edges, then chill another 30 minutes.
Meanwhile, preheat oven to 190 degrees C. Cover the tart base with baking paper and a layer of dry beans, then blind bake 15 minutes. Remove the beans and paper and bake a further 5 minutes, then let cool.
Place the chocolate and butter in a heatproof bowl. Heat the cream to boiling and pour over the chocolate. Add rum. Stir until smooth. Pour into the base, then chill 3 hours or until well set.