In a saucepan on medium heat, heat coconut milk, heavy whipping cream, and kaffir lime leaves until the liquid just begins to boil.
Remove from heat, cover, and let rest for twenty minutes.
Remove the kaffir lime leaves and discard.
Mix in the sugar and corn syrup and return to medium heat, stirring periodically.
When the temperature on a candy thermometer reads 218 F, add the fish sauce.
Continue heating and stirring periodically until it reaches 239 F.
Remove from heat and stir in butter until it is completely melted.
Pour into brownie pans lined with parchment paper or silicone pans. Wait for caramels to cool (about thirty minutes) and then carefully cut into squares using a sharp knife (or if you have $250 to spare, you could use a caramel cutter).
Melt chocolate, being sure to keep it in temper (see my tutorial on how to temper chocolate).
Dip half of each square into melted chocolate and place on parchment paper or a Silpat to cool.
Sprinkle dried lemongrass over chocolate while the chocolate is still warm.