Peel and pit two plums. Slice plums into wedges. Place sliced plums into small bowl of food processor or blender and blend until puréed. This will yield enough purée for 3-4 cocktails.
In an ice-filled shaker, combine all ingredients and shake vigorously for 30 seconds. Strain into rocks glass over ice.
Combine sugar, water and fennel in small saucepan over medium heat. Simmer until sugar is dissolved. Turn off heat and let stand for 30 minutes to allow fennel flavour to develop. Strain out fennel seed. Syrup will keep in airtight container in your fridge for up to two weeks.