In a large skillet, cook the butter and brown sugar over medium heat, stirring, until the sugar dissolves. Place the pineapple in the syrup in a single layer, reduce the heat to medium-low, and cook for 3 to 4 minutes, until browned on the first side. Turn with a small spatula or tongs and cook on the second side until the pineapples are soft and richly browned. Add rum and flambe. Arrange on dessert plates.