MAKE THE TART SHELLS: In a medium mixing bowl, whisk together the flour, cocoa powder, powdered sugar, and salt. Add the cold butter cubes and rub them into the dry ingredients using your fingertips until the mixture resembles fine crumbs. Mix in the egg yolk, then add cold water 1 tablespoon at a time just until the dough comes together without being sticky. Flatten into a disc, wrap in plastic wrap, and refrigerate for 30 to 45 minutes to chill and firm the dough.

SHAPE AND BAKE: On a lightly floured surface, roll out the chilled dough to about ⅛ inch thick. Cut and press the dough into eight 3 to 4-inch mini tart pans. Dock the bottoms with a fork to prevent puffing. Place the tart shells in the freezer for 10 minutes. Bake in a preheated 350°F (175°C) oven for 12 to 14 minutes, or until fully set. Let cool completely in the pans before filling.

MAKE THE GANACHE: Place the chopped dark chocolate in a heatproof bowl. In a small saucepan, heat the cream just until it begins to simmer. Pour the hot cream over the chocolate and let sit for 1 minute. Add the butter and stir gently until the ganache is smooth and glossy. Spoon a thin layer of ganache into each cooled tart shell and refrigerate for about 20 minutes, or until set.

MAKE THE PISTACHIO MOUSSE: Sprinkle the powdered gelatin over the warm milk and let it bloom for 5 minutes. Meanwhile, whip the cold cream with powdered sugar in a mixing bowl until soft peaks form. In a separate bowl, stir together the pistachio paste and vanilla extract until smooth. Warm the bloomed gelatin mixture slightly, then stir it into the pistachio mixture. Add a small drop of green gel food coloring and mix until the color is evenly distributed. Fold the whipped cream into the pistachio base gently, using a spatula, until fully blended. Transfer the mousse to a piping bag fitted with a round tip and refrigerate briefly to firm slightly before piping.

ASSEMBLE THE TARTS: Pipe generous round dollops of pistachio mousse onto the chilled ganache layer in each tart shell. Arrange evenly and refrigerate the tarts for at least 30 to 60 minutes to allow the mousse to set fully before serving.

GARNISH AND FINISH: Just before serving, top the mousse with crushed roasted pistachios and sprinkle mini dark chocolate chunks for added texture and flavor. For a more polished presentation, garnish with white chocolate shavings or a touch of edible gold leaf.