Place the yogurt in a strainer lined with a paper towel and let it drain over a bowl to thicken in the refrigerator for at least 4 hours, and up to 1 day. Discard the liquid and set the thickened yogurt aside.

Puree half the raspberries in a food processor until smooth. Strain into a large bowl, pressing the liquid out with a rubber spatula. Discard the seeds. Whisk in the confectioners' sugar. Stir in the remaining raspberries.

In a chilled small bowl whip the cream until stiff peaks are formed. Gently fold in the yogurt. Fold in the raspberry mixture.

Spoon into cocktail glasses and chill, covered, for at least 1 hour. Serve with ladyfinger cookies.