Place watermelon chunks in a blender and blend until smooth. Strain through a fine-mesh strainer to remove pulp.
Add basil leaves to a cocktail shaker and muddle gently to release oils.
Pour 3 oz of strained watermelon juice, vodka, lime juice, and simple syrup into the shaker. Fill with ice and shake for 15 seconds.
Strain into a Collins glass filled with fresh ice. Top with a splash of club soda.
Garnish with a watermelon wedge and basil leaf.
Notes
he cocktail tastes best with ripe, sweet watermelon. Seeds should be removed before blending.Store extra watermelon juice in an airtight container for up to 2 days.For a mocktail version, replace vodka with additional watermelon juice and club soda.Fresh basil works better than dried basil for this recipe.