Lightly grease an 8 inch (20cm) round tart pan.
In the food processor, blitz the flour, cocoa powder, sugar and salt for a few seconds to mix well. Add in the cold cubed butter and pulse until the pastry starts to come together and form clumps.
Transfer the pastry to the prepared pan and using your fingers, evenly press the pastry up the sides of the tart pan and onto the bottom. Gently pierce the bottom of the crust with the tines of a fork. This is to prevent the pastry crust from puffing up while it bakes.
Cover the tart crust with cling wrap and place it in the freezer for an hour or overnight to firm up. This helps to prevent the crust from shrinking while it bakes.
Preheat oven to 220C (450F). Optional: Just before baking, place a piece of foil or parchment onto the frozen shell and weigh it down with some beans. If baking without weights, simply pierce the base gently with a skewer during baking should the pastry puff up. This will help to release the air that is trapped and flatten out the puffed crust.
Place the frozen tart shell on a larger baking tray and bake the crust at the lowest rack of the oven. Bake for ~15 minutes, then remove the foil with beans and bake for another 5 minutes until golden. If baking without weights, bake for 15 minutes or until golden.
Remove from the oven and set aside to cool. Once cooled, un-mould from pan and chill in the fridge whilst making the filling.