Soak gelatine in cold water. Mix cassis, grape juice and 200 ml water. Squeeze the gelatine, dissolve at low heat (do not boil). Pour into the liquid while stirring constantly.
Cold rinse a box mould (capacity approx. 3/4 litres). Wash the grapes, dab dry, cut in half and remove the seeds. Pour in portions of the grape liquid, placing the individual layers in the fridge in between to set.
Finally, refrigerate again for about 2 hours. To serve, briefly dip the mould in hot water and turn over the jelly. Arrange the jelly in slices. Serve with whipped cream and advocaat.