Applejack cocktails were originally made from the uniquely North American potable applejack, an 80- to 100-proof brandy aged for two years in wood. One might speculate why the applejack cocktail is on the endangered species list, particularly with apple-flavored cocktails so common. The reason is that the majority of modern apple cocktails are made from apple-based spirits other than applejack.
With relatively few true brands of the potent applejack on the market, you are more likely to order a drink that uses apple brandy or schnapps. Barring your own home distillation, seek out real applejack to sample at home.