01 - Preheat oven to 350°F. Position rack in center position.
02 - Place shelled pistachios in a food processor and pulse until finely ground, taking care not to over-process into a paste.
03 - Add flour, granulated sugar, and salt to the ground pistachios. Pulse several times until evenly combined.
04 - Add cold cubed butter and pulse until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
05 - Add egg yolk and pulse until the dough begins to come together in clumps when pressed between your fingers.
06 - Press dough evenly into the bottom and up the sides of a 9-inch tart pan with a removable bottom, building the edges slightly above the rim.
07 - Refrigerate the assembled crust for 20 minutes to firm the butter and prevent shrinking during baking.
08 - Prick chilled crust several times with a fork to release steam. Bake 13–15 minutes until lightly golden. Transfer to wire rack and cool completely.
09 - Pour heavy cream into a small saucepan and heat over medium until it just begins to simmer around the edges. Do not boil.
10 - Place chopped chocolate in a heatproof bowl. Pour hot cream over chocolate and let sit undisturbed for 2 minutes. Add diced butter and vanilla extract.
11 - Stir gently in small circular motions starting from the center outward until ganache is completely smooth, silky, and glossy.
12 - Pour ganache into cooled tart shell, tilting pan to distribute evenly. Smooth surface with an offset spatula.
13 - Refrigerate tart for at least 3 hours or until ganache is firm and set throughout.
14 - Sprinkle chopped pistachios and flaky sea salt over top just before serving. Slice with a sharp knife dipped in hot water for clean edges.