Place a medium saucepan on the grates of the grill, add the butter, and brown sugar, and cook, stirring occasionally, until the sugar has melted and the mixture thickens slightly, about 5 minutes.
While the sauce is cooking, remove the mangoes from the peel and cut into diamond shapes. Place the diamonds onto serving plates.
Remove the pan with the sauce from the heat and add the bourbon. Using a long lit match, ignite the bourbon and allow the flames to subside. Stir in the cinnamon, salt and pecans. Top mangoes with the sauce and with a large dollop of creme fraiche, if using.