Begin by making the tart base. In a large bowl, combine the butter and icing sugar and beat until smooth. Add the egg yolks, one at a time, and mix to combine. Add the cocoa powder, flour, salt and cornstarch and mix just until the dough comes together.
Remove the dough from the bowl and shape into a disc. Wrap the disc in plastic wrap and refrigerate for 30 minutes.
Once the dough has chilled, roll the dough out on a floured surface into a large circle using a rolling pin, aiming to make the dough 1/2 cm thick. Lift the dough into a 9 inch round tart pan without stretching the dough. Ensure that the tart pan is completely covered even up the sides. Trim the excess dough from the top of the pan then prick the bottom of the tart with a fork to allow the steam to release during cooking.
Place the tart pan with the dough back into the fridge for 20 minutes to firm up.
Once chilled, preheat the oven to 350 degrees F. Bake the tart in the center of the oven for 18-22 minutes. Remove the tart from the oven and set aside to let cool completely.
Once the tart shell has cooled, remove it from the tart pan and place it on a serving plate. Set aside while we make the ganache.