Combine the sugar, milk, and butter; boil steadily until the mixture forms a thread when a little is dipped up with a spoon. Add the vanilla; beat until creamy; spread on the cake while it is still warm.

**note: the handwritten recipe claims to be taken from "The Service Cookbook number one" by Mrs. Ida Bailey Allen copyright 1933 (of which I have a copy but I pulled the recipe from the box instead of leafing through this wonderful book).