While working on this tart's chocolate filling, flavored with Irish whiskey and coffee, I had trouble getting the cooked egg yolks and melted chocolate to combine properly. I called Shirley Corriher, food chemist extraordinaire and friend. She walked me through each step to achieve a perfectly smooth and marbled chocolate mixture, which I then poured into a golden pastry shell. This tart is rich and satisfying, so you might want to serve small slices.

To make the crust: Put the flour, confectioners' sugar, and salt in a food processor, and add the butter and shortening. Process, pulsing several times, until the mixture resembles coarse meal. With the machine running, slowly add the water just until moist clumps form. Remove the dough and gather it into a ball; flatten it into a disk. Wrap the dough in plastic wrap and refrigerate it 30 minutes before using. (The dough can be made 1 day ahead; soften it slightly at room temperature before using.)

Roll the dough out on a lightly floured work surface into a 12-inch round. Transfer it to a 9-inch tart pan with a removable bottom. Trim any overhanging dough to 1 inch. Fold the overhanging dough in and press it to itself to form double-thick sides. Prick the bottom of the crust all over with the tines of a fork. Cover and freeze it 30 minutes.

Arrange an oven rack at center position and preheat the oven to 400 degrees F

Bake the crust until it is golden brown, piercing the bottom with a fork if it bubbles, 30 to 35 minutes. Transfer the crust to a cooling rack and cool it to room temperature.

To make the filling: Whisk the granulated sugar, cup of the cream, and the egg yolks together in a medium heat-proof bowl set over (but not touching) a pot of simmering water. Whisk constantly until the sugar has dissolved and small bubbles form around the edge of the bowl, 3 to 4 minutes. Remove it from the heat and stir in the 4 ounces of semisweet chocolate, the unsweetened chocolate, coffee, and 4 tablespoons of the whiskey. Whisk until the mixture is smooth and shiny. Cool it to room temperature.

With an electric mixer on medium-high speed, whip the remaining 1 cup of cream in a medium bowl until soft peaks form. Add the remaining 1 tablespoon of whiskey and beat until the cream is firm. Fold half of the whipped cream into the chocolate mixture, taking care not to blend it completely so that you create a marbled effect. Spread the filling evenly in the cooled tart shell and swirl it with a knife to marble it some more. Refrigerate until the filling is firm, at least 2 hours or up to 6 hours. Cover and refrigerate the remaining whipped cream separately

Serve each slice with a dollop of the reserved whipped cream and, if desired, with a shaving of chocolate.