My great-grandmother made a mean hard sauce. Not only that, many people would like to know how she does it.
Here is one recipe from her files. It may or may not be “the one” but it is tasty.
1/4 cup butter
1 cup powdered sugar
2 tablespoons brandy
2 egg yolks
2 egg whites
1/2 cup cream or milk
Cook over hot water until thickens, add beaten egg whites.