Prepare a campfire and let it cook down to very hot coals or low flame.
Set a small cast iron skillet on a grate over the campfire. Add the butter and let it melt, about 2 minutes.
Stir in the brown sugar, vanilla, and cinnamon. Cook, stirring constantly, until the mixture comes together and is thickened and bubbly, 3 to 5 minutes. It will seem like the butter and sugar aren't coming together at first, but just keep stirring!
Pour in the rum and cook for 1 minute.
Stir in the bananas and cook, stirring to coat them with the sauce, until they are slightly softened, about 2 minutes. Serve immediately over pound cake (or vanilla ice cream), if desired.
Notes
Scaling--If you are doubling the recipe, use a 10-inch skillet.
Keep it flat--Keep the skillet as level as possible when cooking so that the sauce cooks evenly.
Manage the heat--If the sauce begins to boil too rapidly or begins foaming while you are cooking, remove it from the heat or move it to a cooler part of the grate.
Dietary--This recipe is suitable for gluten-free diets, but double check your brand of spiced rum to make sure any flavoring additives are gluten free.