Sift the flour and Cocoa Powder into a bowl and stir in the sugar. Add the butter and rub into the dry ingredients until well blended and the mixture resembles a fine crumble topping.
Add the egg yolk and mix together. Use your hands to bring the mixture together to form a dough then turn on to a lightly floured surface and knead gently until smooth.
Roll out the pastry to fit a 2cm (3/4inch) deep, 24cm (9 ½ inch) diameter loose-bottomed, fluted flan tin. Place the pastry in the flan tin and trim to neaten the edges, lightly cover with cling film and chill for 30 minutes.
Ten minutes before baking, preheat the oven to 200°C (180°C Fan, Gas Mark 6). Line the centre of the tart with a piece of baking parchment and fill the centre with baking beans or raw rice.
Put the tin on a baking tray and bake for 15 minutes. Carefully remove the paper and the beans or rice, prick the base and return to the oven for 7-8 minutes until the pastry is firm and cooked through. Leave to cool then carefully remove from the tin and place on a serving plate. Carefully spread the jam over the bottom of the pastry case and set aside.
Break 275g of Extra Dark Chocolate into pieces and leave to one side. Place the double cream into a sauce pan and gently heat over a low heat until just simmering. Remove the cream from the heat and add the Extra Dark Chocolate. Leave the chocolate to melt without stirring for a minute, then begin to stir the chocolate through the cream until it has fully melted.
Spoon the chocolate ganache into the pastry case, smooth the top and chill for at least 2 hours to set.
To serve, using a vegetable peeler, shave off small pieces of the reserved Extra Dark Chocolate on top. The tart is now ready to serve and enjoy!