Peel banana; cut in half crosswise, then cut each half in half lengthwise.
Arrange banana pieces and pineapple slices in a single layer in a 9- or 10-inch pie pan.
Sprinkle with sugar and dot with butter.
Broil about 6 inches below heat until fruit is glazed (about 6 minutes), basting a few times.
Spoon hot fruit into 2 dessert dishes and top each serving with sour cream; pour any remaining hot butter sauce from pan over sour cream.