01 - Preheat oven to 350°F.
02 - Place pistachios in a food processor and pulse until finely ground.
03 - Add flour, sugar, and salt to the ground pistachios. Pulse to combine evenly.
04 - Add cold cubed butter and pulse until the mixture resembles coarse crumbs.
05 - Add egg yolk and pulse until a dough begins to come together.
06 - Press the dough evenly into the bottom and up the sides of a 9-inch tart pan with a removable base.
07 - Refrigerate the crust for 20 minutes to firm up before baking.
08 - Prick the chilled crust several times with a fork. Bake for 13-15 minutes or until lightly golden. Let cool completely.
09 - In a small saucepan over medium heat, warm the heavy cream until just simmering.
10 - Place chopped chocolate in a heatproof bowl. Pour hot cream over the chocolate and let sit for 2 minutes. Add butter and vanilla extract.
11 - Stir gently until the ganache is smooth and glossy.
12 - Pour the ganache into the cooled tart shell. Smooth the top evenly with a spatula.
13 - Refrigerate for at least 3 hours, or until the ganache is fully set.
14 - Garnish with chopped pistachios and a sprinkle of flaky sea salt before serving.