Thaw the raspberries in a colander, then puree and bring to the boil. Then caramelize the sugar in a saucepan and add the butter. Add the still warm raspberry puree and bring to the boil again. Add the vanilla sugar and the cassis.
Keep warm over a low heat until dessert is served. Can also be prepared the day before and reheated. Cinnamon parfait tastes very good with it.