Sprinkle the gelatine over 1/4 cup of dark rum and allow to soften for 5 minutes.
Meanwhile, stir together 1 1/4 cups of whipping cream, brown sugar and espresso powder in a saucepan over medium-high heat. Bring to a simmer stirring until brown sugar has dissolved. Remove from heat then whisk in gelatine mixture until dissolved.
Whisk in the coconut milk, vanilla essence and sour cream until smooth. Evenly divide the mixture between eight 3/4 cup custard cups or moulds then cover each with plastic wrap and chill at least 4 hours to overnight.
Bring 3/4 cup whipping cream and golden syrup to a simmer over medium-high heat. Once simmering remove from heat and stir in the chocolate until melted and smooth; about 2 minutes. Stir in 1 tablespoon of rum and set aside.
To serve, run a knife around the edge of each mould then set each cup into a shallow bowl of hot water for 10 seconds to loosen. Invert the mould over a serving plate and remove the panna cotta. Spoon chocolate sauce around each panna cotta and garnish with a sprig of mint.