Combine butter, ¼ cup whipping cream and egg yolks in top of double boiler. Whisking constantly, cool over simmering water for 10 min. Remove from heat and stir in chocolate chips; stir until smooth. Transfer to a small bowl and cool to room temperature.. Stir in liqueur.
Combine egg whites, sugar, water and cream of tartar in top of double boiler. Whisking constantly, cook over simmering water for 9 min. Transfer to a large bowl and beat until stiff peaks form. Gently fold chocolate mixture into egg white mixture.
Spoon mousse into small serving dishes. Loosely cover. And chill until whipped cream mixture is prepared or ready to serve.
Place a medium bowl, and mixer beaters in freezer until well chilled.
In chilled bowl. Whip remaining ¾ cup of whipping cream until soft peaks form. Add confectioners sugar and orange extract; beat until stiff peaks form.
When ready to serve, garnish with whipped cream mixture and if desired well drained mandarin orange slices.