01 - Preheat oven to 350°F. Position the rack in the center of the oven.
02 - Add shelled pistachios to 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 to the food processor and pulse until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
05 - Add the egg yolk and pulse just until the dough begins to come together. Avoid overworking.
06 - Transfer the dough to a 9-inch tart pan with a removable bottom. Press evenly across the base and up the sides to create a uniform crust about 1/4 inch thick.
07 - Refrigerate the 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 the edges are lightly golden. Transfer to a wire rack and let cool completely.
09 - Pour the heavy cream into a small saucepan and set over medium heat. Bring to a gentle simmer—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 soften.
11 - Add the diced butter and vanilla extract to the bowl. Stir gently in small circles starting from the center and working outward until the ganache is smooth, glossy, and fully emulsified.
12 - Pour the ganache into the completely cooled tart shell. Use an offset spatula to smooth the surface into an even layer.
13 - Refrigerate the tart for at least 3 hours, or until the ganache is firm to the touch and holds a clean slice.
14 - Sprinkle chopped pistachios over the surface and finish with a light dusting of flaky sea salt. Slice into wedges and serve chilled or at cool room temperature.