- Crust:
- 3 cups chocolate wafer crumbs
- 10 tablespoons butter, melted
- Filling:
- 1 pound semisweet chocolate, chopped
- 2 eggs
- 4 eggs, separated
- 2 cups heavy whipping cream
- 6 tablespoons confectioners' sugar
- Topping:
- 2 cups heavy whipping cream
- 1/4 cup confectioners' sugar, or to taste
- 2 tablespoons orange liqueur (such as Grand Marnier®) (optional)
- Mix chocolate wafer crumbs and butter together in a bowl. Press over the bottom and up the sides of a 10-inch springform pan. Freeze until firm, 20 to 30 minutes.
- Melt chocolate in top of a double boiler over simmering water, stirring frequently and scraping down the sides with a rubber spatula, about 5 minutes. Remove from heat.
- Whisk 2 eggs and 4 egg yolks in a bowl. Gradually pour in melted chocolate, whisking constantly, until well blended.
- Beat egg whites in a glass, metal, or ceramic bowl until stiff peaks form.
- Beat cream in a chilled glass or metal bowl with an electric mixer until foamy. Add 6 tablespoons confectioners' sugar; continue to beat until soft peaks form.
- Fold one-third of the egg whites and one-third of the whipped cream into the chocolate mixture until lightened. Fold in remaining egg whites and whipped cream until thoroughly incorporated. Pour into the crust.
- Chill cake until mousse is set, 6 hours to overnight.
- Combine 2 cups heavy cream, 1/4 confectioners' sugar, and orange liqueur in a large bowl; beat until stiff peaks form. Spoon into a pastry bag fitted with a star tip. Pipe whipped cream rosettes to garnish each slice.