Rim a chilled rocks glass with lime juice, then dip into crushed graham crackers.
In a shaker, gently muddle mint with simple syrup and key lime juice.
Add white rum, heavy cream, and a handful of ice. Shake about 10 seconds until chilled and lightly frothy.
Strain into the prepared glass filled with fresh ice.
Top with a splash of club soda. Garnish with a mint sprig and a thin key lime wheel.
Notes
Fresh key lime juice pops the most; regular lime works in a pinch. Skip the cream and add more club soda for a lighter take. Rim just before pouring so the graham stays crisp. Use crushed ice for a slushier, beach-bar vibe.