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Mix the butter with the sugar and treacle, then add the rum. Make a split through the inside curve of each banana skin – be careful not to cut all the way through. Spread a spoonful of the spiced butter inside each skin.
Place the bananas, curved-side up – so the butter doesn’t leak when it melts – onto individual sheets of foil large enough to wrap them completely. Crimp up the foil to make a parcel, then put onto the embers for 15 mins, turning once. The bananas should be sticky and caramelised, but check one and cook for longer if necessary. Serve with a scoop of ice cream.