Step 1

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.

Step 2

Roll out dough to an 11-inch round, 1/4 inch thick 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.

Step 3

Bake until firm, about 30 minutes. Let cool completely; reduce oven temperature to 300 degrees.

Step 4

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 pastelike. Spread evenly into bottom of cooled tart shell, pressing firmly with the bottom of a measuring cup until flat and smooth.

Step 5

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.