Preheat the slow cooker if necessary; see the manufacturer's instructions. Add the butter, sugar and lime rind and juice to the warming slow cooker pot and stir until the butter has melted.
Slit the vanilla pod along its length, open it out with a small sharp knife and scrape the tiny black seeds away from inside the pod. Add the seeds and the pod or vanilla extract, if using, to the slow cooker pot along with the rum and boiling water.
Add the bananas to the slow cooker pot, arranging them in a single layer and pressing them beneath the liquid as much as you can. Cover with the lid and cook on low for 1½–2 hours or until the bananas are hot.
Spoon the bananas and rum sauce into dishes and decorate with extra lime rind curls and scoops of vanilla ice cream, if liked.