First, make the pie dough. Sift the flour, unsweetened cocoa, confectioners' sugar, and salt into a mixing bowl and make a well in the center. Put the butter and egg yolk in the well and gradually mix in the dry ingredients, using a pie dough blender or two forks. Knead lightly and form into a ball. Cover with plastic wrap and chill in the refrigerator for 1 hour.
When chilled, unwrap the dough. Preheat the oven to 350°F. Roll out the dough on a lightly floured workcounter. Use it to line a 12x4-inch rectangular tart pan and prick the pastry shell with a fork. Line the shell with parchment paper and fill with dried beans. Bake for 15 minutes. Remove from the oven, remove the beans and parchment paper and set aside to cool.
To Make the Filling, put the cream and jelly into a pan and bring to a boil over low heat. Remove the pan from the heat and stir in the chocolate until melted and smooth. Stir in the butter until melted and smooth. Pour the mixture into the pastry shell and set aside to cool.
Put 1 1/3 cups of the blackberries, the lemon juice, and the superfine sugar into a food processor and process until smooth. Strain through a fine mesh strainer. Transfer to a bowl and stir in the cassis. Set aside.
Remove the tart from the pan and place on a serving plate. Arrange the remaining blackberries on top and brush with a little blackberry and liqueur sauce. Serve the tart with the sauce on the side.