In medium saucepan, combine egg yolks and milk. Heat over low heat, stirring constantly, until mixture is almost boiling and is thickened enough to coat spoon. Remove from heat. Stir in chocolate chips, vanilla and liqueur; continue stirring until chocolate is melted. Cool to room temperature, stirring occasionally. Beat egg whites until stiff; fold gently into the chocolate mixture. Fold in whipped cream. Refrigerate until ready to use. Fill cupcake with icing.