Add all ingredients to a medium sized pot and bring to a simmer. Place lid on the pot and let simmer for about 10 minutes. (It’s important to keep the lid on or too much of the water will dissolve.)
Transfer the contents to a small glass jar and place in the refrigerator to cool.
Combine the sugar, flaky sea salt, and lime zest on a plate. Rim both glasses with a lime wedge, then coat the rim with the sugar mixture and fill with crushed ice.
In a cocktail shaker or glass jar, lightly muddle the mint with a cocktail muddler or the back of a wooden spoon until aromatic. Add the rum, lime juice, simple syrup, a handful of ice cubes and shake.
Strain into the two glasses, top with a splash of seltzer water and garnish with lime slices and more mint.
If you've tried this recipe, come back and let us know how it was!