With pineapple on its side, cut off a thick, lengthwise slice (do not include green crown).
With a grapefruit knife, remove fruit from both pieces of shell and cut into bite-size chunks; reserve deeper shell.
Sprinkle pineapple to taste with sugar and stir in rum.
Mound in shell and dot with butter.
Wrap completely (including crown) in foil.
Bake in a 350° oven for 20 minutes.