My mom made the plum pudding hard sauce, but she made it into a mound that could be plopped by the spoonful onto the pudding. It looked "hard" but was soft like whipped butter. Any one familiar with this recipe/ how to make it?

Jo, is it possible that your mother whipped heavy cream for the topping? And maybe seasoned it with nutmeg, etc?
The texture "plopped by the spoonful" comments make me think of homemade whipped cream. :)
Hope you find out and are able to make a recipe like your mom's. :)

It sounds like your mom actually made brandy butter which is very similar to hard sauce, but can be molded into a "mound" or other shape and kept in the refrigerator for quite a long time. It's basically butter, sugar, brandy, and flour or corn starch. Delicious!