Chocolate Chestnut Mascarpone Tart

YIELD: 12 servings. In a food processor fitted with the metal chopping blade, combine the flour, sugar, salt, and cocoa powder. Pulse the machine eight to nine times to blend. Scatter the butter over the flour mixture and pulse the machine until the butter is cut into the flour and the mixture resembles coarse meal. Add the yolks and continue to process in on/off pulses only until the mixture is evenly incorporated and the particles begin to hold together. Scrape the dough onto a work surface and form it into a ball. Flatten it into a disc and wrap in plastic wrap. Chill 1 hour. Position a rack in the center of the oven and preheat to 350 degrees F. Remove the chilled disc from the refrigerator. Place the dough between two pieces of plastic wrap and roll the dough into a small round. Lift and rotate the dough one quarter turn after each roll. Continue rolling until the circle measures approximately 14 inches in diameter and is abut 1/8 inch thick. Remove the top layer of plastic wrap. Carefully roll the dough around the rolling pin and transfer it to a 12-inch fluted tart pan with a removable bottom. Unroll the dough into the pan. Lift the edges of the dough and gently press the dough into the bottom and up the sides of the pan. Trim off any excess dough. Refrigerate the dough for 20 to 30 minutes, until firm. Bake the tart shell 20 to 30 minutes or until set. Place on a wire rack and cool completely. Place the chopped chocolate in a medium bowl and set aside. In a non-corrosive medium saucepan bring the sour cream, heavy cream and 1/4 cup of sugar to a boil over medium-high heat. In a large bowl using a hand-held electric mixer, beat the eggs, egg yolks, cornstarch and the remaining 1/4 cup of sugar at medium speed until pale. Whisk one-third of the hot cream mixture into the egg mixture and return the whole mixture to the pan. Cook over medium-high heat while constantly stirring with a whisk for 3 to 5 minutes or until thick. Pour the thickened mixture over the reserved chocolate and whisk until incorporated. Scrape the mixture into the prepared crust and smooth the top with a rubber spatula. Chill in the refrigerator for 2 hours. In a 4 1/2-quart bowl of a heavy-duty electric mixer, using the wire whip attachment, combine the mascarpone, heavy cream, chestnut puree, confectioners' sugar and vanilla. Beat on medium-high speed until soft peaks form. Place the mixture in a pastry bag fitted with a medium star tip and pipe in a shell pattern covering the top of the chilled tart. Refrigerate the tart for 1 hour before serving.