This filling is from The Wedding Cake Book by Dede Wilson. She also has a recipe for straight raspberry buttercream filling, but it is quite lengthy. I will be happy to post it, if this is not what you are looking for. Just post me.

Light Raspberry Mousse

Make & refrigerate the mousse at least one day, even up to 3 days, ahead. This gives it time to firm. You can also make this to serve as a low fat dessert on its own, maybe with fresh raspberries sprinkled on top. Defrost the raspberries in the refrigerator overnight, in the bags in a bowl of warm water, or on low in the microwave.

Makes about 10 cups.

Three 12 oz pkg individually quick frozen, unsweetened raspberries
3 Tbsp unflavored gelatin (one packet is just short of 1 tablespoon)
1 cup cold water
1 cup cranberry-raspberry juice
3/4 cup superfine sugar
3 cups lowfat vanilla

Defrost the raspberries. Place the berries & any juice in the container of the food processor fitted with the metal blade & puree for about 15 seconds. Strain out the seeeds, if desired.

Meanwhile, sprinkle the gelatin over the cold water in a small pan & stir to combine. Let sit 5 min. until the gelatin has softened. Add the cranberry-raspberry juice & heat on very low heat. Stir until the gelatin dissolves. Do not boil. Add the sugar & continue to heat, without boiling, until the sugar dissolves. Stir it a few times to facilitate the process.

Remove from the heat & pour into a bowl that is set in a larger bowl filled with ice & ice water. Add the puree & whisk into the gelatin mixture. Cool the mixture, stirring over the ice until it is as thick & viscous as an egg white, for about 30 minutes.

When the mixture is thick, pour off any liquid whey from the yogurt & stir the yogurt into the gelatin mixture gently, but thoroughly, with a whisk. Cover the bowl & chill overnight or store in an airtight container in the refrigerator.