Hard sauce is a sweet, rich dessert sauce made by creaming or beating butter and sugar with rum (rum butter), brandy (brandy butter), whiskey, sherry (sherry butter), vanilla or other flavourings.

In the United Kingdom, brandy butter and rum butter are particularly associated with the Christmas and New Year season and Christmas pudding and warm mince pies, serving as a seasonal alternative to cream, ice cream or custard.

Rum butter specifically is typically found in Cumbria and is not common in other regions of the UK, while brandy butter is found nationwide and is a more commonplace Christmas accompaniment.

[1] Though it is called a sauce, it is neither liquid nor smooth, with a consistency more akin to whipped butter.

Under European Community regulations, to be called rum/brandy/sherry butter, it must contain at least 20% butterfat.