Strain into a coupe glass. Garnish with grapefruit twist.
Combine water and cardamom pods in a small saucepan over medium heat. Bring to a boil.
Add sugar and stir until dissolved. Reduce heat and simmer for 15 minutes.
Remove from heat and transfer to a mason jar. Store in the fridge overnight and then strain off the cardamom pods. Syrup will keep well in the mason jar (with lid on) for 2 weeks.