If someone handed you a bottle of Angostura bitters, what would you do with it? Shake it in your spag bol, à la Lea & Perrins? Apply it as hair ointment to a balding dome? Although a bottle of bitters is a staple in bars, the herbalised liquor is often overlooked, taking a humble back seat to the glossy bottles of craft gin and aged rum. But it’s a crucial wonder ingredient, and more and more bartenders are brewing their own.