Cut the pineapple into 8 wedges, cutting away and discarding the core. Melt the butter and sugar in a nonstick frying pan over a medium heat. Stir continuously until the mixture starts bubbling, then turn up the heat and add the pineapple wedges. Cook for 3 minutes on each side or until just caramelised. Add the rum and cook for 1 minute more. Remove the pan from the heat and set aside to cool. Meanwhile, stir together the fromage frais, lime and icing sugar. Divide the pineapple between 4 plates and serve with a generous dollop of fromage frais.