01 - Preheat oven to 350°F. 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 flour, granulated sugar, and salt to the ground pistachios. Pulse several times until evenly combined.
04 - Add 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 until the dough just begins to come together when pinched between your fingers.
06 - Transfer the dough to a 9-inch tart pan with a removable bottom. Press it evenly across the base and up the sides to form a uniform shell.
07 - Refrigerate the pressed 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 to 15 minutes until lightly golden. Let cool completely in the pan on a wire rack.
09 - Pour the heavy cream into a small saucepan and warm over medium heat until it just begins to simmer around the edges. 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.
11 - Add the diced butter and vanilla extract to the bowl. Stir gently in small circular motions starting from the center 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 firmly set and no longer jiggles when gently shaken.
14 - Sprinkle the top with chopped pistachios and a light dusting of flaky sea salt. Slice into wedges and serve chilled or at cool room temperature.