First of all, Make the pastry by beating the butter and icing sugar together until smooth. add the egg yolks one at a time and mix until combined.
Sift together the flour, cocoa powder, and cornstarch in a separate bowl. Mix this with the butter mixture until well combined. Form the dough into a disk, wrap it in plastic and let cool before using.
To soften the chocolate pastry dough, knead it once or twice on a lightly floured surface. After this, roll the dough out to a rectangle less than 1/4 inch thick.
The dough should be lifted carefully and placed on a fluted tart pan with removable bottom. Then press the dough into the shell. Trim any excess. Use a fork to transfer the pastry. The tart shell should be chilled for at least 20 min.
Preheat the oven to 350 F (125 C). Bake the tart shell chilled on a baking tray for 18 minutes until it is dry and has a dull, even finishes. Let it cool down while you make the filling.
Pour the cream and butter over the chocolate and let it cool for a few seconds. Now Use a spatula to gently stir the mixture until it melts. Next, add the sugar and coffee. After this, In a small bowl beat the eggs lightly.
After this, Add the egg mixture to the chocolate and whisk gently until it is incorporated. Bake the mixture in the tart shell for 12 minutes at 350 F/175 C. Let the tart cool to room temperature and chill for at least two hours before serving.
You can store the tart in the refrigerator for up to two days.