For the tart shell, pulse the flour, powdered sugar, and salt in a food processor. Add chilled butter and pulse until the mixture resembles coarse crumbs. Add egg yolk and pulse until the dough begins to form. If needed, add ice water to bring it together. Wrap the dough and refrigerate for 30 minutes.

1. Roll out the dough to about ¼-inch thickness. Press it into a tart pan, trim the excess, and dock the bottom with a fork. Preheat the oven to 350°F (175°C) and bake with parchment paper and weights for 10-12 minutes. Remove the weights and bake for 5-7 more minutes until golden. Cool completely.
2. For the crèmeux, toast pistachios if using raw. Blend them until finely ground. In a saucepan, heat cream, milk, and half the sugar until it steams. Whisk egg yolks and remaining sugar until smooth. Gradually add hot milk mixture to egg yolks to temper. Cook over medium heat until thickened, about 5 minutes. Stir in pistachios, butter, and vanilla until smooth. Refrigerate for at least 2 hours until set.
3. Once the crèmeux is chilled, fill the tart shell with it, smoothing the surface with a spatula. Garnish with chopped pistachios and optional chocolate shavings or berries.
4. Chill the tart for at least an hour before serving to set the filling.