*Make the plum syrup: Add everything to a small saucepan. Bring gently to a simmer and cook for about 10 minutes, pressing the fruit lightly as it softens. Remove from the heat, leave to cool, then fine strain. Keep refrigerated.
Add the bourbon, lemon juice, plum syrup, egg white and bitters to a cocktail shaker without ice.
Shake vigorously for about 15 seconds to build that lovely foam.
Add ice and shake again until thoroughly chilled.
Double strain into a chilled coupe or over fresh ice in a rocks glass.