1. Lightly spray a grill and heat until hot. Place pineapple slices on the grill and allow to cook 3-4 minutes on each side. (When it is time to flip, you should see caramelized grill marks on the slice.)
2. Meanwhile, in a small saucepan, bring the butter, rum and sugar to almost a full boil. Reduce heat and simmer until the mixture is slightly thick and can coat the back of a spoon, about 10 minutes. Whisk occasionally to insure that it doesn't separate. Spoon sauce over pineapple and serve warm.