Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking dish.

Prepare the cake batter according to the package directions.

In separate bowls, toss the blueberries and raspberries with cornstarch. If using frozen berries, keep them frozen.

Fold the blueberries into the batter first, followed by the raspberries, using a gentle hand to avoid crushing the fruit.

Pour the batter into the prepared baking dish and spread evenly.

Bake for 35–45 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs. Cover loosely with foil if the cake browns too quickly. Frozen berries may require up to 50 minutes.

Let the Summer Berry Sheet Cake cool completely.

Beat the softened cream cheese with ½ cup powdered sugar until smooth.

In another bowl, whip the heavy cream, vanilla, and remaining powdered sugar until stiff peaks form.

Fold the whipped cream into the cream cheese mixture until smooth and fluffy.

Spread the frosting over the cooled cake.

Drizzle strawberry or raspberry topping over the frosting and swirl with a knife.

Chill before serving and garnish with