For the pastry, whizz the plain flour, cocoa powder, caster sugar, salt and butter in a food processor until evenly distributed and resembling fine breadcrumbs. Add the yolks & vanilla and pulse until combined. It will be crumbly - tip out onto a sheet of cling film and bring together with your hands using the cling film to help you wrap it tightly around. Form into a large circle and refrigerate for 30 minutes.
When you’re ready to roll out, pop two pieces of non-stick baking parchment down on the surface and roll out the pastry between the pieces as it can be quite sticky due to the cocoa powder.
Line a loose bottomed tin with the pastry, making sure it is tucked well into the corners. Trim off the excess overhang, leaving 2cm or so, then fold the residual overhang underneath itself to give yourself a double layer rim. Crimp with your fingers or a fork, then chill until extremely cold (you can even cook it from frozen) - this takes a few hours.
Preheat the oven to 180°C, scrunch up some non-stick baking parchment and place into the tart case, fill with ceramic baking beans and cook for 20 minutes until the edge feels dry to the touch - then remove the parchment and beans and cook for a further 10 minutes until the base feels sandy to the touch - it can be hard to tell when chocolate pastry is cooked through so you’ve got to use your nose, when it smells delicious, it’s probably ready. Allow to cool on a wire rack.
To make the ganache, heat the double cream until steaming hot.
Meanwhile, break up the chocolate and add to a large heatproof boil.
Once the cream is steaming (not boiling) pour it over the chocolate and allow it to rest for 1 minute before stirring gently from the centre outward - you want to incorporate as little air as possible and over-agitating the chocolate can lead to graininess or splitting.
Once the chocolate is fully incorporated, add the butter and salt and stir again to combine.
Allow to cool slightly, then pour into your prepared chocolate pastry tin. Refrigerate for 30 minutes before decorating.
Decorate with lightly whipped cream, berries, grated chocolate and anything else that takes your fancy!