A Sweet Manhattan with cognac substituted for the whiskey.
Named after and likely created at New York's Delmonico's Steakhouse Restaurant, the history of which dates back to 1837 when the Delmonico brothers opened "the first fine dining restaurant in the country." There is still a Delmonico's in New York City but the restaurant owned by the Delmonico family at 44th Street and Fifth Avenue closed in 1923.
In his 1948 The Fine Art of Mixing Drinks, David Embury handily informs us that a Delmonico "is a plain Manhattan with cognac substituted for the whisky. If orange bitters are used instead of Angostura, it becomes a Harvard [not according to others]. This is sometimes also called a Saratoga. The real Saratoga, however, is made with apple brandy. There is also a Delmonico Special which is merely a Medium Martini with 1 teaspoon of brandy to each drink and a twist of orange peel."