01 - Preheat oven to 350°F. Position the rack in the center of the oven.
02 - Place the 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 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 evenly across the bottom and up the sides to create a uniform layer.
07 - Refrigerate the shaped crust for 20 minutes to allow the butter to firm up and prevent shrinking during baking.
08 - Prick the chilled crust several times with a fork to release steam. Bake for 13 to 15 minutes until the edges are lightly golden. Remove from the oven and let cool completely.
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 circles starting from the center and working outward until the ganache is smooth, glossy, and completely emulsified.
12 - Pour the ganache into the completely cooled tart shell, using a spatula to smooth the surface into an even layer.
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 light dusting of flaky sea salt. Slice into 8 portions and serve chilled or at cool room temperature.