Heat oven to 350°F (325°F for dark or nonstick pan). In 13x9-inch pan, melt butter in oven. Sprinkle brown sugar evenly over butter. Sprinkle pineapple over brown sugar.
Using reserved pineapple juice and additional pineapple juice to measure 1/2 cup minus 1 T. Add 1 T. rum to juice.Mix cake mix,pineapple juice-rum mixture,eggs and butter as directed on box. Pour batter over pineapple .
Bake 42 to 48 minutes (44 to 53 minutes for dark or nonstick pan) or until toothpick inserted in center comes out clean. Immediately run knife around side of pan to loosen cake. Place heatproof serving plate upside down onto pan; turn plate and pan over. Leave pan over cake 5 minutes so brown sugar topping can drizzle over cake; remove pan. Cool 30 minutes. Serve warm or cool. Store covered in refrigerator.