01 - Preheat the oven to 350°F. Position the rack in the center of the oven.
02 - Place the shelled pistachios into a food processor and pulse until they are 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 when pinched between your fingers.
06 - Transfer the dough to a 9-inch tart pan with a removable bottom. Press it firmly and evenly across the bottom and up the sides to form a uniform crust.
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 the crust is 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 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 completely emulsified.
12 - Pour the ganache into the completely cooled tart shell, using a spatula to smooth the top 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 chopped pistachios and a pinch of flaky sea salt over the top just before serving. Slice into 8 wedges and serve chilled or at cool room temperature.