Sprinkle the gelatine over water and let it bloom. The water will absorb the gelatine, becoming a thick mass.
Next, run a knife down the vanilla bean (if using), making a slit. Do not cut it in half, only one side should be split - kind of like opening a pod of peas.
Position the knife horizontally and drag it down the open vanilla bean to obtain the seeds (essentially you're scraping the bean).
In a medium sized pot, combine the coconut milk and cream. To this, add the sugar, both, the vanilla bean and the vanilla seeds (if using) and the lime rind. Let the mixture come to a boil.
At this stage, add in the gelatine mixture, stirring till it has dissolved. At this point, you can add in the vanilla extract if using.
I like to strain the mixture, but keep in mind that gelatine sets very quickly (agar-agar sets even quicker). You can also remove the vanilla bean if using and pour it directly into the ramekins.
Divide the mixture between the ramekins. You'll need around ½ - ⅔ of a cup of liquid in each portion to make 4 servings.
Cover each ramekin with cling film, and refrigerate until firm, I would say 4 hours at the least, or even better, overnight.
To release the panna cotta from its mould, dip each ramekin into hot water for around 3 seconds, then run a knife around the edges to loosen it.
Invert the panna cotta onto a plate.
Serve the desserts diced mango and grate a bit of lime zest for that final finish.
This panna cotta is a great make-ahead dessert (in that you can make them a day or two in advance) and leave them in the fridge, then garnish right before serving.