You can make the chocolate-wafer crust and custardy chocolate filling up to a day in advance. Spread with whipped cream and garnish with chocolate curls just before serving. Martha made this recipe on episode 613 of Martha Bakes.
Make the crust: Preheat oven to 350 degrees. In a food processor, pulse wafers until fine crumbs form. Add butter, sugar, and salt, and process until well combined. Press mixture firmly into bottom and up sides of a 9-inch fluted tart pan. Bake until crust is fragrant, about 15 minutes. Let cool completely on a wire rack.
Make the filling: In a medium saucepan, heat milk, egg yolks, sugar, and cornstarch over medium-high, whisking occasionally, until simmering. Reduce heat to medium and add chocolate. Continue to cook, stirring constantly, until bubbling and thick, 4 to 5 minutes (about 2 minutes after it comes to a boil). Add salt and remove from heat. Stir in vanilla or bourbon. Let cool about 10 minutes, whisking 2 or 3 times.
Pour custard into baked and cooled crust. Press a piece of plastic wrap directly on surface of custard. Refrigerate until filling is firm, 4 hours or up to 1 day.
Make the topping: With an electric mixer on medium-high speed, whisk cream just until soft peaks form. Add confectioners' sugar and whisk until stiff peaks form. Spread whipped cream over custard. Garnish tart with chocolate curls just before serving.