Put the frozen raspberries into a blender with the sugar, and puree. You may have to be patient as they will be difficult to blitz at first since they are extremely hard when frozen.
Spoon 1 to 2 teaspoons of the raspberry puree into each glass, top with 2 tablespoons of the yogurt and sprinkle a layer of cookie crumbs on top.
Cover the crumbs with 1 more layer of yogurt, then raspberry puree and finally cookie crumbs again.
These can be prepared the night before, in which case don't sprinkle the final layer of cookie crumbs until you are ready to eat the swirls but leave the glasses with the yogurt on top, covered with plastic wrap.
Recipe courtesy of Nigella Lawson(Copyright 2004, Feast, Hyperion, All Rights Reserved)