Food processor: Place flour, cocoa powder, sugar, and salt in a food processor, and pulse a few times to mix. Add the icy butter cubes and process until the mixture resembles breadcrumbs.
Add egg and process very shortly to combine. The pastry should now start clumping together, and it will hold easily when pressed.
Refrigerate: Quickly press the dough into a ball without working it too much. Flatten into a disk, wrap in plastic foil and refrigerate for 30 minutes.
By hand: Mix the dry ingredients in a large bowl. Add the very cold butter, and rub it into the flour with the fingertips until the mixture resembles breadcrumbs, working as quickly as possible to prevent the dough from becoming warm. Add the lightly beaten egg, and using a cold knife, stir until the dough binds together. Wrap and refrigerate as instructed above.
Chop the chocolate very finely with a chef's knife or in a food processor.
Gently heat the cream and the instant coffee in a small saucepan until it starts to simmer. Don't let it come to a boil. Stir often to make sure that the coffee is completely dissolved.
Stir: Remove from the heat and stir in the chocolate until the mixture is completely smooth. Stir in the Bailey's.
Cool: Transfer to a mixing bowl, let cool shortly on the counter, then transfer to the fridge and refrigerate for one hour.
Lightly grease the bottom of a tart tin with removable bottom (about 23 cm/ 9 inches diameter).
Roll the dough on the lightly floured surface into a circle slightly larger than the tin. Line the tin with the shortcrust. Press the pastry into the edges and up the side of the tin. Trim the excess dough. Prick with a fork all over.
Refrigerate: Place in the fridge or even better freezer while you preheat the oven to 180 degrees Celsius/ 350 degrees Fahrenheit.
Bake blind: Place a piece of parchment paper on top of the crust. Fill the tin with dry beans and bake for about 15 minutes. Carefully remove the beans and the parchment paper. Continue baking for a further 5 minutes or until cooked through. Don't overbake, or the crust will break when you cut the tart.
Let cool completely before filling; it will not take long.
Carefully transfer the pastry shell onto a serving platter.
Stir the jam until smooth and spread it on the base of the tart.
Whip the chocolate cream until fluffy. Immediately spread it onto the tart and smooth it with a spoon or a spatula. Work quickly; the cream will stiffen again quite fast.
Sprinkle evenly with unsweetened cocoa and decorate as desired.