Fill a cocktail shaker with ice. Add all ingredients, minus the salt, and shake vigorously.
Dip the rim of the glass in extra lemongrass syrup, then dip in the Hawaiian black salt.
Strain the Bangkok margarita into the prepared cocktail glass and serve!
For Lemongrass Syrup: Smash a stalk of lemongrass with a muddler to help release some of the oils. Chop up the lemongrass and place it in a pot with one cup of sugar and one cup of water. Bring the syrup to a simmer so that the sugar is fully dissolved and remove from heat. Let the lemongrass steep in the syrup until it is completely cool Strain out the lemongrass and bottle.