"People say bitters are the salt and pepper of the bar, but really, they're like the spice rack," says Brad Thomas Parsons about one of the most essential, and misunderstood, cocktail ingredients. In his book Bitters, he examines the intensely flavoured concoctions and how they're made (usually from high-proof alcohol infused with fruits, spices, roots and barks like gentian and cinchona).

Sometimes still marketed as digestive aids, bitters were once sold as patent medicines; they later became a key ingredient in classic drinks like the Manhattan and the old-fashioned. Today's craft-cocktail enthusiasts are resurrecting 100-year-old "lost" bitters recipes and creating out-there new flavours, such as Mexican mole.

To understand how bitters can enhance a drink, Parsons suggests mixing a Manhattan with and without them: "One will be beautiful, and the other will be overly sweet and cloying." Here, he shares recipes for bitters, inspired by what he calls "the holy trinity"—Angostura (woodsy and spiced), Peychaud's (anise-scented) and orange—along with recipes for cocktails and dishes that use them.

In this video I'll show you how I'm making my bitters for my favourite cocktail - old-fashioned.