The traditional accompaniment for plum pudding, hard sauce is made by beating butter, sugar and flavoring together until smooth and creamy. The sugar can be powdered, granulated or brown. The flavoring is generally brandy, rum or whiskey, though vanilla or other extracts may also be used. This mixture is refrigerated until "hard" (the texture of butter). It's often spooned into a decorative mold before chilling and unmolded before serving. Hard sauce is known in England as brandy butter.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.