Position oven rack about 8 inches away from the broiler unit and preheat the broiler to 500 degrees F.
Butter the bottom of an 11 by 7-inch baking dish and arrange the banana slices over the butter in 1 even layer. Set aside.
In a small bowl, combine the brown sugar, cinnamon, orange zest, nutmeg, and orange juice, and stir to combine. Spoon the sugar mixture evenly over the bananas, using the back of a spoon to make sure the bananas are evenly coated with the mixture. Transfer to the oven and broil until golden brown on the edges and syrupy, 6 to 8 minutes.
Remove the baking dish from the oven and drizzle lightly with rum, if desired. Set aside to cool briefly before serving. Serve the warm sauce and bananas over vanilla ice cream.
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