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 flour, granulated sugar, and salt to the ground pistachios. Pulse several times until evenly combined.
04 - Add the cold cubed butter 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 in clumps. Do not overwork.
06 - Transfer the dough to a 9-inch tart pan with a removable bottom. Press firmly and evenly across the base 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 lightly golden. Transfer to a wire rack 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 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 circular motions starting from the center 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 evenly.
13 - Refrigerate the tart for at least 3 hours, or until the ganache is firm to the touch and set throughout.
14 - Sprinkle the top with chopped pistachios and a light pinch of flaky sea salt. Slice into wedges and serve chilled or at cool room temperature.