Preheat the oven to 200C and butter the tart dish.
In a food processor (unless you’re good at making it by hand), combine the flour, cocoa, the icing sugar and salt and give it a slight whiz to mix. Then add the butter chunks and pulse until the dough comes together in a big clump.
Remove the blade, grab the dough and transfer to the tart dish. Use your fingers press and stretch the dough patiently to all directions until you have an as equally thick as possible shell. You can use the back of a wooden spoon to shape it.
Put in the oven (middle shelf) and bake for 10’-12’.
Take it out of the oven and place on a flat and heatproof area. By this time it will look like the dough has slid back. There will probably be bubbles. Take a fork or toothpick and gently poke a few holes to release the steam; with the back of a wooden spoon press the dough back to reshape the shell. We’re not looking for a geometrically perfect shell, ladies and gentlemen. Just make sure the dough is evenly distributed around. Usually the corners come out thicker – just a heads up!
Put back in the oven and bake for an extra 5’. Take it out and if required, repeat the fork/wooden spoon process, if needed for the last time. Then let it cool down completely, ideally by an open window. By the time you’ll make the ganache it will be good to go.