As previously tweeted:
.5oz batavia arrack
1oz fresh lime juice,
splash rhubarb bitters
muddled fresh basil (best to muddle right in the cocktail shaker if you have a long enough pestle or a short enough shaker)
Shake the above hard as you can with ice and pour over ice into a tall glass leaving about 20-25% extra space. Top up with ginger beer. Sip in the sun.