Creamy Hard Sauce

From 1922:

One-half cup butter, 1 cup powdered sugar, ¼ cup thin cream, ¼ teaspoon vanilla.
Work butter until creamy and shiny like satin. Add sugar gradually and beat until both are light and creamy. Add cream a little at a time and continue beating. Add vanilla and place in a cold place. This is very good when brown sugar is used in place of the white powdered sugar.