Words are copied and pasted from the Martha Stewart site:
Photographs all my own…
Coat a 9-inch fluted round tart pan with a removable bottom with cooking spray. Make the crust: Whisk together flour, cocoa powder, chopped pistachios, and 1/4 teaspoon salt. Beat butter on medium speed until fluffy, about 3 minutes. Add sugar, and beat until pale, about 2 minutes. Reduce speed to low, and add vanilla, then flour mixture; beat until just combined and dough holds together when pressed between 2 fingers. Press dough into a disk, and wrap in plastic. Refrigerate for 15 minutes.
Roll out dough to 1/4-inch thickness on a lightly floured surface. Press dough into bottom and up sides of tart pan. Trim excess dough flush with edges of pan using a knife. Refrigerate until firm, at least 1 hour or overnight. Meanwhile, preheat oven to 325 degrees.
Meanwhile, make the pistachio paste: Pulse pistachios, sugar, and 1/4 teaspoon salt in a food processor until mixture begins to clump together. Add oil, and pulse until smooth and paste like. Spread evenly into bottom of cooled tart shell, pressing firmly with the bottom of a measuring cup until flat and smooth.
Make the filling: Place chocolate in a heatproof bowl. Heat cream and milk in a saucepan over medium heat until almost boiling.Pour over chocolate. Let stand for 2 minutes. Slowly whisk until smooth. Let cool for 10 minutes. Stir in egg until combined. Place tart shell on a baking sheet, and pour in filling. Bake until just set, 30 to 35 minutes. Let cool. Garnish with finely chopped pistachios.