Prepare the Pie Crust: In a food processor, combine flour, powdered sugar, and salt. Pulse until mixed. Add cold cubed butter and pulse until coarse crumbs form. Add egg and pulse until dough comes together. Shape into a disk, wrap in plastic, and chill for 1 hour. Preheat the oven to 375°F (190°C). Roll dough and fit it into a 9-inch pie plate. Line with parchment and weights, blind bake for 15 minutes. Remove parchment and bake for another 5-7 minutes until golden. Let cool.

Make the Pistachio Filling: Grind pistachios into a fine paste. In a saucepan, whisk sugar, cornstarch, and salt. Add eggs and whisk smooth. Gradually add milk and cook over medium heat until thickened. Remove from heat and stir in pistachio paste, almond extract, and butter. Pour into cooled pie crust and chill for at least 4 hours.

Prepare the Whipped Cream: Whip heavy cream, powdered sugar, and vanilla extract until soft peaks form. Spread over chilled pistachio filling.

Assemble the Pie: Garnish with chopped pistachios and serve.