Process raspberries and strawberries in batches in a food processor or blender until smooth. Transfer to a food mill or push through a fine sieve into a large measuring cup or bowl. Add enough water to the strained berries to equal 5 cups liquid.

Whisk cornstarch and water in a small bowl. Whisk the cornstarch mixture and granulated sugar into the strained berries and transfer to a Dutch oven or large saucepan. Cook over medium heat, whisking constantly (make sure to whisk the bottom well), until the mixture is very hot and beginning to bubble. Cook for 1 minute more, whisking constantly.

Transfer the pudding to a large bowl and let cool for 10 minutes. Press a piece of plastic wrap directly onto the surface and chill in the refrigerator for at least 2 hours (it will continue to thicken as it chills).

Whip cream in a small bowl until soft peaks form. Add confectioners' sugar and vanilla; continue whipping until firm peaks form. Fold in yogurt. Serve the pudding with a dollop of cream on top.

Ingredient Note: Greek-style yogurt is made by removing the whey from cultured milk. Removing the whey gives it an extra thick and creamy texture--making it the perfect ingredient for a lower-fat dessert topping. You can strain regular yogurt to make it thick like Greek-style yogurt. Line a sieve with cheesecloth and set it over a bowl. (Alternatively, use a coffee filter lined with filter paper.) Spoon in 1 cup nonfat plain yogurt and let it drain in the refrigerator until reduced to 3/4 cup, about 2 hours.

MAKE AHEAD TIP: The pudding (Steps 1-3) will keep for 2 days, the cream (Step 4) will keep for up to 8 hours. Store separately, covered, in the refrigerator.