Add watermelon chunks and mint leaves to a cocktail shaker, then muddle gently to release juices and mint oils.
Pour in bourbon, simple syrup, and lemon (or lime) juice.
Fill the shaker with ice and shake for about 15 seconds until well chilled.
Strain into a rocks glass filled with fresh ice (fine-strain for a smoother texture).
Optional: Top with a splash of club soda for a lighter drink.
Garnish with a mint sprig or a small watermelon wedge.
Notes
Adjust simple syrup based on the sweetness of your watermelon. Always use fresh citrus. You can prep watermelon juice ahead and keep it cold for faster service.