In a mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, powdered sugar, and salt. Cut in the chilled butter using a pastry cutter until it resembles coarse crumbs, then mix in the egg yolk and just enough cold water to form a dough. Press the dough evenly into a tart pan and chill it in the refrigerator for at least 30 minutes.

Preheat your oven to 350°F (175°C). Line the tart shell with parchment paper and fill it with pie weights or dried beans. Bake in the preheated oven for 15-20 minutes, or until set. Carefully remove the weights and parchment, and return to the oven for an additional 5-10 minutes, until the crust is slightly crisp. Allow it to cool completely.

In a large mixing bowl, whip the heavy cream until stiff peaks form. In another bowl, combine the mascarpone cheese, powdered sugar, cooled espresso, vanilla extract, and coffee liqueur, mixing until smooth. Gently fold the whipped cream into the mascarpone mixture until fully incorporated.

Spoon the luscious espresso mascarpone filling into the cooled tart crust, smoothing the top with a spatula. Place the tart in the refrigerator to chill for at least 2 hours, or until the filling is set.

Just before serving, dust the tart with cocoa powder and garnish with chocolate shavings and coffee beans, if desired. Slice, serve, and enjoy this delightful treat that will surely impress your guests!