Prepare the Pie Crust:
In a food processor, combine the flour, powdered sugar, and salt. Pulse to mix. Add the butter and pulse 5-7 times, until the butter is broken down into small pebbles.
With the motor running, add the egg and process until the dough forms a cohesive ball.
Roll out the dough on a flour-dusted surface to an 11 circle and line a 9 pie plate. Crimp the edges as desired and prick the bottom with a fork. Freeze the crust for at least 15 minutes.
Preheat the oven to 375°F (190°C).
Line the chilled crust with parchment paper and fill with pie weights (or dry rice/beans). Bake for 20 minutes, remove the parchment and weights, and bake for an additional 10 minutes or until fully cooked through. Set aside.
Prepare the Pistachio Filling:
Add the pistachios to a food processor or high-speed blender. Process until the pistachios release their oil and form a thick paste (pistachio butter), about 5-10 minutes.
In a medium pot, whisk together the sugar, cornstarch, and salt. Add the eggs, pistachio butter, and milk, whisking until fully combined.
Heat the pot over medium, whisking constantly, and cook until the pudding thickens and large bubbles form, about 8 minutes.
Remove from heat and strain the mixture through a fine mesh sieve to remove any lumps.
Stir in the butter and almond extract until fully combined.
Pour the pudding into the baked pie crust and smooth the top. Let it cool at room temperature before chilling in the fridge for at least 3 hours or overnight.
Assemble the Pie:
In a mixer with a whisk attachment, beat the cream cheese, sugar, and vanilla until smooth (about 2 minutes). Add the heavy cream and beat until soft stiff peaks form, about 2 more minutes.
Spread the whipped cream evenly over the cooled pie and garnish with chopped pistachios.
Serve immediately or keep chilled for up to 1 day before serving.