The richness of blackcurrant syrup (25ml crème de cassis works well as a substitute) is balanced by the sourness of the vinegar with the liquorice and...
I had to perform some substitutions, but the result was superb and I'll gladly credit the original recipe with that. The integration of flavors between rum, cassis, vinegar, liquorice and salt worked stunningly well. I used Flor de Caña 7y rum, Giffard Cassis Noir de Bourgogne, and about 1/2 tsp or 2.5ml of Duval pastis (strong liquorice taste) in place of grated liquorice.