This recipe is one of our desserts to give thanks for.
Line shell with parchment paper and fill with rice or pie weights. Bake 10 to 12 minutes; remove rice and paper and bake until crust is cooked through, 8 to 10 more minutes. Cool on a rack.
Whisk half-and-half, yolks, brown sugar, cornstarch, cocoa, and salt in a medium saucepan. Cook over medium-high heat, stirring constantly, until it thickens and bubbles. Cook 1 minute more; remove from heat. Stir in bittersweet chocolate, butter, and vanilla. Pour into pie shell; cool completely. Transfer to refrigerator to set, at least 2 hours or up to 2 days.
When serving, make a topping by beating heavy cream, pistachio paste, and sugar (if using) in a cold bowl to soft peaks. Sprinkle pie with pistachios and chocolate shavings (if using); add a dollop of pistachio cream to each slice.
Note: If you buy pistachio paste specifically to make this recipe (which doesn't require much), use the rest for breakfast (smeared on toast with honey) or as a snack (mixed with jelly on crackers).