For the panna cotta, in a saucepan over a medium heat combine the coconut milk and double cream, then stir in sugar and lemongrass. Heat until just below boiling point, then set aside to cool.
Soak the gelatine in cold water for 5 minutes to soften.
Squeeze the excess water from the gelatine and whisk into coconut mixture until melted and combined. Sieve the mixture through a fine sieve and pour into dariole moulds or small cups and chill for 2 hours or until set.
Cut the pineapple into thin wedges and set aside. In a pan, melt the butter then add the sugar and rum and stir until the sugar is dissolved. Bubble until slightly thickened then add the pineapple wedges and cook until soft and slightly caramelised.
Take out of the pan and serve alongside the panna cotta, with some of the caramel drizzled on top. Finish with a squeeze of lime and some toasted coconut flakes.