Copyright (2023) from A History of the World in 10 Dinners: 2,000 Years, 100 Recipes (Rizzoli)
This drink is described as one of a group of “fancy cocktails,” and I recall it being described as “very popular with the ladies,” egg yolk and all. The recipe does not include exact measurement, but the ingredients can be eyeballed satisfactorily.
About 2 jiggers maraschino liqueur
1 egg yolk
About 2 jiggers vanilla cordial
About 1 ounce cognac
Fill a glass roughly halfway with the maraschino. Using a long-handled bar spoon, carefully slip in the egg yolk without breaking it. Invert the bar spoon and very slowly pour the vanilla cordial (about the same amount as the maraschino) into the bowl of the spoon and down the side of the glass, so that it forms a layer on top of the maraschino. Do the same with the cognac, but add just enough to cover the surface of the vanilla. Serve.