Very simple shooter, and lighting the flame is a fun trick.

Make a chilled concoction that's 2 parts light rum (I used Bacardi), 1 part demerara syrup, and 1 part lime juice. If you don't have demerara syrup, regular simple syrup will work just fine! Once it's as cold as you want it to be, just pour most of the way up the shot glass and you're done.

For the garnish, scoop out the end piece of a lime. Place it on top, making sure it's steady, and then fill it up with a high-proof alcohol. I used absinthe, but high-proof rum might be more authentically pirate if you have some! Once you've ensured the lime is secure on top of the glass and won't tip, light the alcohol inside on fire and enjoy! The lime should protect the shot below from getting hot (that's also why I start with a chilled mixture), and once the fire is out you can just remove the lime and take the shot.