The myth is reported by Pliny.
This is the story: a certain Kora (also called Callirhoe), was in love with a boy at Corinth who had to leave the country. Whereupon she drew on the wall the outline of the shadow of his face. From this outline her father Butades modeled a face in clay, and baked the model, thus preserving for his daughter a face in relief of the boy she loved.
My thoughts? Amazing, but, couldn’t Butades just as well have made a death mask of the face of the boy?