01 - In a large mixing bowl, whisk together the all-purpose flour, cocoa powder, powdered sugar, and fine sea salt until evenly distributed.
02 - Add the cold cubed butter to the flour mixture. Using a pastry cutter or your fingertips, work the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
03 - Add the egg yolk and 2 tablespoons of cold water to the crumb mixture. Gently fold and press the dough together until it just holds its shape. Add the remaining tablespoon of water only if the dough remains too dry and crumbly.
04 - Shape the dough into a flat disc, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes to relax the gluten and firm the butter.
05 - On a lightly floured surface, roll the chilled dough out to an 11-inch round, about ⅛ inch thick. Carefully transfer it into a 9-inch tart pan with a removable bottom. Press the dough firmly into the corners and fluted edges, then trim away any overhang. Prick the base evenly with a fork and chill for 15 minutes.
06 - Preheat the oven to 350°F. Line the chilled crust with parchment paper and fill with baking weights or dried beans. Bake for 15 minutes. Remove the weights and parchment, then continue baking for 8 to 10 minutes until the crust looks dry and set. Allow it to cool completely in the pan.
07 - Place the finely chopped dark chocolate in a heatproof bowl. Heat the heavy cream in a small saucepan over medium heat until it just begins to simmer around the edges. Immediately pour the hot cream over the chocolate and let it sit undisturbed for 2 minutes.
08 - Starting from the center, stir the chocolate and cream together in small circles until the mixture becomes completely smooth and glossy. Stir in the softened butter and vanilla extract until fully incorporated and the ganache has a silky consistency.
09 - Pour the finished ganache into the cooled tart shell, using an offset spatula to smooth the surface into an even layer.
10 - Scatter the roughly chopped pistachios evenly across the surface of the ganache. Finish with a light pinch of flaky sea salt if desired.
11 - Refrigerate the assembled tart for at least 2 hours, or until the ganache is firmly set and no longer jiggles when gently shaken.
12 - Remove the tart from the pan and place it on a serving plate. Dip a sharp knife in hot water, wipe it dry, and slice cleanly through the tart for neat, professional edges. Reheat and wipe the knife between each cut.