In a 4 cup glass measure, blend the eggnog and cornstarch until smooth. Microwave uncovered at high for 4 5 minutes, until mixture comes to a boil and thickens, stirring twice. Cover and let cool slightly; refrigerate until cold. In a bowl, beat whipping cream until soft peaks form; beat in rum or brandy. Fold into eggnog mixture until smooth. Refrigerate sauce until ready to serve. Makes about 2 cups. From The Gazette 90/12/19.