Make the mousse: Line a 7- to 8-inch bowl with plastic wrap, leaving a 2-inch overhang. Beat 1 cup heavy cream in a large bowl with a mixer on medium-high speed until stiff peaks form, about 2 minutes. Refrigerate until ready to use. Clean and dry the beaters.
Put the chocolate in a large heatproof bowl. Heat the remaining 1 cup heavy cream in a small saucepan over medium heat until hot but not boiling, then pour over the chocolate and stir until melted. Set aside.
Bring 1 1/2 inches of water to a boil in a wide pot; reduce the heat to maintain a gentle simmer. Whisk the egg yolks, 1/2 cup orange liqueur and 1 tablespoon granulated sugar in a medium metal bowl. Set the bowl over the pot of simmering water (do not let the bowl touch the water). Continue whisking, frequently scraping down the sides of the bowl with the whisk, until the mixture is thick, has almost doubled in volume and leaves a trail when you lift the whisk, about 5 minutes. Remove the bowl from the pot. (If the yolk mixture thickens too quickly or starts to scramble, remove the bowl from the pot and let cool slightly, whisking, then return to the pot.)
Put the egg whites and the remaining 3/4 cup granulated sugar in a large metal bowl and set it over the simmering water. Beat on medium speed with a hand mixer until fluffy and shiny, about 3 minutes. Remove the bowl from the pot and continue beating until stiff peaks form, about 4 more minutes.
Using a rubber spatula, fold the egg yolk mixture into the melted chocolate mixture until combined, then fold in the beaten egg whites until combined with some white streaks. (Do not overmix.) Gently fold in the whipped cream until no streaks remain. Fold in the remaining 1 tablespoon orange liqueur. Scrape the mixture into the plastic wrap¿lined bowl. Cover with plastic wrap and freeze at least 6 hours and up to 2 days.
Make the chocolate shell: Combine the chocolate and coconut oil in a medium microwave-safe bowl; microwave in 30-second intervals, stirring, until smooth. Remove the mousse from the freezer and uncover. Invert onto a platter and gently remove the plastic wrap. Pour the chocolate shell over the mousse and spread with the back of a spoon or a pastry brush to make a thin layer. Return to the freezer until the chocolate hardens, at least 10 minutes. Dust with confectioners' sugar right before serving.