There are Many Nightcaps, but Only One Classic "Night Cap
The Night Cap is another fun find from in W.C. Whitfield's 1939 Just Cocktails bartending guide. It is very simple to remember: equal parts brandy, anisette, and curacao with an egg yolk. The mix of flavors is quite intriguing and very fitting for a cocktail from that era.
Anisette is called for in the original recipe, though many of us do not keep it in stock, so you can use any of the other anise-flavored liqueurs such as absinthe in its place.
See Also
Chrysanthemum
Morning Glory
Saratoga Brace-Up