Add watermelon chunks and mint leaves to the shaker. Muddle until the watermelon releases its juice.
Pour in the white rum, lime juice, and simple syrup.
Fill the shaker with ice and shake for 15-20 seconds.
Strain the mixture into a Collins glass filled with fresh ice.
Garnish with a watermelon wedge and mint sprig.
Notes
The drink tastes best with ripe, sweet watermelon. Pick a watermelon that feels heavy for its size and has a yellow spot where it sat on the ground.Store leftover watermelon chunks in an airtight container for up to 3 days.For a mocktail version, replace the rum with coconut water or club soda.Double-strain the cocktail if you want to remove small watermelon pieces.