Cut skin off pineapple slices then cut each slice into quarters across the centre. Cut out the hard centre piece from each quarter then halve each quarter. Halve, stone, peel and cut the mango into bite-sized cubes.
Put the lime juice, butter, rum and granulated sweetener into a small pan, stir well and heat on low for a minute or until the butter melts. Add the fruit, stir and leave for 10 minutes, turning the fruit once or twice.
Remove fruit from the pan with a slotted spoon and thread onto four small pre-soaked wooden kebab sticks. Heat the grill and grill the kebabs for 3-4 minutes, turning once, or until lightly golden.
Tip any pan juices into the rum sauce pan and heat until bubbling - serve the kebabs with the sauce.
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