Place the water, sugar, Malibu liqueur and the agar-agar in a saucepan then add the lime juice. Bring to the boil, whisking constantly.
Pour the jelly into a mould and allow to set in the fridge for at least 30 minutes.
Peel the grapefruits, making sure to remove all of the pith then cut each segment in 2. Place the segments in a bowl, add the whisky and the sugar. Allow to marinate for at least 10 minutes.
Preheat the oven to 180°C.
Remove the Carambar toffees from their wrappers and place on a baking sheet. Transfer to the oven and cook for 10 minutes. Make sure to leave a little space between each Carambar as they will spread out when cooked.
«Make the jelly a day in advance to make sure that it is well set. The longer you marinate your grapefruit for, the stronger the whisky flavour will be.»