After writing my previous post, I did a bit more research into the puzzling Coupe Yvette au Sherry on the menu.
First clue: From the New York Public Library, I found a menu from the Savoy Hotel, London, that lists Coupe Yvette aux Paillettes as the second of eight courses at a dinner to celebrate American Independence Day, 1925. This suggests that it was served between the canapés and the entrée at the Savoy in that era. Paillettes, I believe, are straw-like wafers, a bit like crispy strips of crêpes.
Second clue: The same NYPL menu archive also has mentions of Coupe Yvette in menus from the Hotel New Yorker, 1937 – where it's listed under Ice Cream – and the Fifth Avenue Restaurant, 1917 – where it's with the desserts. This suggests that perhaps it is a sorbet-type dish, that might have been served as a palate-cleanser between the courses.
That would go very well in my steampunk style menu, as it sits between the oysters and salmon and a chilled soup. The question is, can I make a sorbet using Creme Yvette and sherry?
I also found this recipe, from a book called The Hostess of To-Day by Linda Hull Larned, in the Sherbets chapter:
No. 391. Creme Yvette
A : 1 pt. Lemon Ice No. 639 (Water Ice) : B : 4 tbsps. Creme Yvette
cordial : C : 18 fresh violets. Add B to A before freezing, (ill
glasses, add 2 or '■'• of C to each glass.