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, keeping face clear of flames. Arrange on dessert plates.