Put the mango pieces, granulated sugar and root ginger into a medium-sized saucepan and add 100ml water. Stir and bring to a simmer for 7-8 mins or until the mango is starting to soften. Take off the heat and allow to cool for 10-15 mins.
Pour the cooled mango syrup through a sieve into a cocktail shaker. Add the lime juice, Pisco and egg white. Fill to the top with ice, shake well, then strain into 4 tumblers. Add a couple of shakes of Angostura bitters to each one and serve.