Sift together the Flour, Cacao Powder, Icing Sugar and Salt, place in a large mixing bowl and mix.
Add the cold Butter cut into small cubes. Mix it with the dry ingredients by rubbing it between your fingers (or using a Pastry Blender) until you get very fine crumbs (see note 1).
Add the Egg and mix it by hands in until you get a smooth, soft dough (see note 2).
Place between two sheets of baking paper and roll into a large circle with a rolling pin, about 3 mm or 1/8 inch thick. Dust with a little bit of flour if very sticky. Place over a flat tray and leave in the fridge to rest for at least 1 hour - or up to 24 hours.
Line your Tart Ring or Tart Pan (see note 3) and dock the bottom of the pastry by pricking it with a fork. Place back in the fridge for at least 1 hour - or up to 24 hours.
Preheat your oven on 160'C. Put the Tart Ring/Pan with the Pastry in the freezer while the oven is preheating, then bake for 30 to 40 minutes or until fully baked. Set aside to cool down completely.
Chocolate Ganache Filling
Finely chop the Cooking Chocolate (if using a chocolate bar and not chocolate callets/chips) and place it in a large heat-proof mixing bowl.
Pour the Cream into a small saucepan and heat up on low heat until it starts to simmer. Pour half of the hot cream over the chopped Chocolate and leave for 2 to 3 minutes without touching it. Then using a stiff spatula or silicone spatula, stir together in small circular movements, starting from the centre of the bowl then making your way towards the edges (see note 4).
Pour the rest of the hot cream over the ganache (see note 5) and repeat the process, stirring in gentle circular movements until all the chocolate has melted and the cream has been incorporated. You should get a smooth, shiny ganache.
Directly pour the warm Ganache in the baked Tart Crust. If needed, use a small offset spatula or the back of a spoon to spread it out. Place in the fridge to set for at leat one hour, or until the ganache has hardened.
Optionally: when fully set, decorate with your favourite toppings like a melted chocolate drizzle, fresh berries, cacao nibs, desiccated coconut etc,...
I highly recommend using the measurements in Grams/Ml instead of Cups/Spoons for more accuracy and a better result.I used a Perforated Tart Ring that is 2 cm high / 24 cm wide or 0.75" high/ 9.75" wide. This pastry recipe will make more crust than needed to fill this tart ring, so you could use a slightly larger, or higher tart pan instead.For all detailed instructions, tips, troubleshooting and food processor method on the pastry, read my Chocolate Tart Crust recipe.Ingredients Notes:(*) Chocolate: I used a 70% Dark Chocolate, but you can use any of your preferred chocolate. Simply make sure you are using Cooking Chocolate - not eating chocolate. Eating Chocolate will not melt properly and might result in a grainy, unset ganache. There is no added sugar to the ganache, but you can adjust the level of sweetness by choosing a sweeter type of chocolate.Instructions Notes:
You should not see any loose powders anymore. You can also use a Food Processor to make the Pastry if preferred.
You might find it easier to transfer the dough over your kitchen bench to mix in the egg and 'knead' it in. Try not to overwork the pastry at this point.
You might need to leave it at room temperature for a few minutes if the pastry is very cold and hard to handle. Alternatively, if the pastry seem to become too soft, place it back in the fridge for a few minutes.
The chocolate won't be fully melted at this point - it is normal. Gently mix until all the cream has been incorporated into the chocolate.
If the cream seems to have cooled down, reheat it up to a simmer before adding the second half. If you still have some unmelted chocolate after adding the second half of the cream, either place the ganache in the microwave (for no longer than 30 seconds at the time, stirring well each time until fully melted) or preferably over a double boiler.