Sprinkle the unflavoured gelatine over 50ml of dark rum and allow to soften for 5 minutes.
Meanwhile, stir together 300ml of whipping cream, dark brown soft 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 extract and soured cream until smooth. Evenly divide the mixture between eight 175ml ramekins, cups or moulds, cover each with cling film and chill for at least 4 hours to overnight.
Bring 175ml 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 ramekin, then set each ramekin into a shallow bowl of hot water for 10 seconds to loosen. Invert the ramekin over a serving plate and remove the panna cotta. Spoon chocolate sauce around each panna cotta and garnish with a sprig of mint.