01 - In a large mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, powdered sugar, and fine sea salt until evenly blended.
02 - Add the cold, cubed unsalted 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 pea-sized pieces remaining.
03 - Add the egg yolk and 2 tablespoons of cold water to the crumb mixture. Gently mix until the dough just begins to come together, adding the remaining tablespoon of water only if needed. Shape the dough into a flat disc, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes.
04 - On a lightly floured surface, roll the chilled dough out to a circle approximately 11 inches in diameter. Carefully transfer it into a 9-inch tart pan with a removable bottom, pressing the dough evenly into the corners and up the sides. Trim any excess dough from the edges and prick the base several times with a fork. Refrigerate for another 15 minutes.
05 - Preheat the oven to 350°F. Line the crust with parchment paper and fill it with baking weights or dried beans. Bake for 15 minutes, then remove the weights and parchment. Continue baking for 8 to 10 minutes until the crust appears dry and set. Allow it to cool completely in the pan.
06 - 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 stand undisturbed for 2 minutes.
07 - Beginning at the center, stir the cream and chocolate together in small circles, gradually widening until the mixture is completely smooth and glossy. Stir in the softened butter and pure vanilla extract until fully incorporated.
08 - Pour the finished ganache into the completely cooled tart shell, using an offset spatula to smooth the surface into an even layer.
09 - Scatter the roughly chopped pistachios evenly across the surface of the ganache. Finish with a light pinch of flaky sea salt if desired.
10 - Refrigerate the assembled tart for at least 2 hours, or until the ganache is firmly set. To serve, remove the tart from the pan and slice with a hot, clean knife for neat, sharp edges.