Prepare and bake white cake mix in two 9-inch cake pans according to package directions. Let cool.

To make the yoghurt mixture, combine cream, sugar, and gelatin in a large non-aluminum saucepan. Gently cook over low heat until gelatin is dissolved, about 5 minutes. Remove from heat and let cool to room temperature.

Whisk yoghurt and preserves into cream mixture until well combined. Gently fold in blueberries and raspberries.

Place one of the cooled 9-inch cakes on a cake plate. Spread the yoghurt mixture on the cake. Place the second cake layer on top and chill 4 hours or overnight.

Prepare topping: Beat 2 cups heavy cream until it begins to thicken. Add powdered sugar and vanilla, and continue beating until stiff.  Spread topping on top of chilled cake and decorate with additional berries, if desired. Serve chilled and refrigerate leftovers.

Makes 12 servings.