01 - Preheat oven to 350°F (175°C). Position the rack in the center of the oven.
02 - Place shelled pistachios in a food processor and pulse until finely ground, being careful not to over-process into a paste.
03 - Add the all-purpose flour, granulated sugar, and salt to the ground pistachios. Pulse several times until evenly combined.
04 - Add the cold cubed butter and pulse until the mixture resembles coarse crumbs with a few pea-sized pieces remaining.
05 - Add the egg yolk and pulse until the dough just begins to come together when pinched between your fingers.
06 - Press the dough evenly into the bottom and along the sides of a 9-inch tart pan with a removable base, ensuring a uniform thickness throughout.
07 - Refrigerate the shaped crust for 20 minutes to firm up the butter and prevent shrinking during baking.
08 - Prick the chilled crust several times with a fork to allow steam to escape. Bake for 13–15 minutes until lightly golden. Let cool completely on a wire rack.
09 - Pour the heavy cream into a small saucepan and heat over medium heat until just simmering — small bubbles should form around the edges but do not let it boil.
10 - Place the chopped dark chocolate in a heatproof bowl. Pour the hot cream over the chocolate and let it sit undisturbed for 2 minutes to allow the chocolate to melt.
11 - Add the diced butter and vanilla extract to the chocolate mixture. Stir gently in small circular motions starting from the center outward until the ganache is smooth, glossy, and emulsified.
12 - Pour the ganache into the completely cooled tart shell. Smooth the surface evenly with an offset spatula, working from the center outward.
13 - Refrigerate the tart for at least 3 hours, or until the ganache is fully set and firm to the touch.
14 - Sprinkle the top with chopped pistachios and a pinch of flaky sea salt just before serving. Slice into 8 even wedges with a sharp knife.