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.