1. In a large saucepan, combine the cream, sugar and cornstarch. Cook and stir over medium heat until mixture reaches 160° or is thick enough to coat the back of a metal spoon.
2. Stir a small amount into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir for 2 minutes; stir in chocolate until melted. Remove from the heat; gently stir in vanilla.
3. Cool to room temperature, about 15 minutes. Fold in whipped topping. Place 1/4 cup each in four parfait glasses.
4. Combine blueberries and raspberries; place 1/4 cup in each glass. Repeat layers of chocolate mixture and berries twice. Cover and refrigerate for at least 1 hour. Garnish with strawberries and whipped topping if desired. Yield: 4 servings.