Roll the shortcrust pastry dough onto a tart pan. Poke with a fork all over the bottom and sides. Chill in the freezer for 15 minutes, then bake in the pre-heated oven at 180c (356F) for 20 minutes.
Add the water and granulated sugar to a pot, set on medium-high heat and bring to a boil, stirring to dissolve the sugar. When it boils, add the cocoa and instant coffee, constantly stirring to prevent lumps. Cook for 1 minute.
Pour the coffee chocolate cream onto the freshly-baked shortcrust, spreading it out to cover the whole bottom. Let cool completely (placing it over a wire rack will speed up this process, plus you can also refrigerate it as soon as it cools a bit).
In a bowl, whip the cream to hard peaks. In another bowl, add the egg yolks and powdered sugar and whisk until fluffy and pale. Add the mascarpone to yolk mixture and mix until homogeneous, about 1 minute.
Add the whipped cream to the mascarpone mix and fold it in working with a spatula with gentle upward movements. When the pie crust has cooled completely, spread this cream all over the cocoa and coffee layer.
Chill in the fridge for a couple of hours to let the mascarpone cream set. Just before serving, sprinkle cocoa powder on top to give the pie a classic tiramisù finish.
Notes
Store airtight in the refrigerator for up to 4 days.