Mix the mascarpone with the yoghurt until smooth, fold in the whipped cream.
Put the frozen raspberries in a large bowl or casserole dish. Spread the mascarpone cream evenly and sprinkle thickly with the dark sugar. Place the food in the refrigerator overnight until serving - this is important so that the raspberries can thaw and the sugar forms a nice, firm crust.
Note: The amount of sugar sounds huge, but it is necessary because the remaining ingredients do not contain any sugar.