Prepare the Tart Shell:

• Preheat your oven to 350°F (175°C).
• In a bowl, combine the flour, powdered sugar, and salt. Add the cold cubed butter and cut it into the dry ingredients until the mixture resembles coarse crumbs.
• Mix in the egg yolk until the dough comes together. Form the dough into a disk, wrap it in plastic, and chill in the refrigerator for 20 minutes.
• Roll out the dough on a lightly floured surface and press it into a 9-inch tart pan with a removable bottom. Prick the base with a fork and chill for an additional 10 minutes.
• Line the crust with parchment paper, fill with pie weights or dried beans, and bake for 15 minutes. Remove the weights and parchment, then bake for another 5 minutes until just set. Remove from oven and let cool slightly.

Make the Pistachio Filling:

• In a food processor, blend the pistachios with the granulated sugar until they form a coarse meal.
• In a small saucepan over low heat, combine the heavy cream and softened butter until melted and smooth. Remove from heat and stir in the pistachio paste, vanilla extract, and lemon zest if using.
• In a separate bowl, whisk the egg yolks. Slowly temper the egg yolks by gradually adding the warm cream mixture, whisking constantly. Once incorporated, stir in the blended pistachio mixture until smooth.

Assemble and Bake:

• Pour the pistachio filling into the pre-baked tart shell, smoothing the top with a spatula.
• Bake in the preheated oven for 20–25 minutes, or until the filling is set but still has a slight wobble in the center.
• Remove from the oven and allow the tart to cool completely on a wire rack. Chill in the refrigerator for at least 1 hour before serving to enhance the flavors and texture.

Serve:

• Once chilled, remove the tart from the pan and slice into wedges. Garnish with a dusting of powdered sugar or a few chopped pistachios if desired.