Creating a Mixed Berry Icebox Cake is a simple process. Follow these easy steps:

1. Prepare the Berries: Wash the mixed berries. Slice the strawberries into halves or quarters, depending on size. Keep the blueberries, raspberries, and blackberries whole.

2. Make the Cream: In a large bowl, whip the heavy cream until soft peaks form. Gradually add the powdered sugar and vanilla extract, continuing to whip until firm peaks form.

3. Layer the Cake: In a 9×9-inch baking dish, start with a layer of graham crackers, covering the bottom completely.

4. Add Cream: Spread a generous layer of whipped cream over the graham crackers.

5. Add Berries: Distribute a mix of berries evenly over the cream.

6. Repeat Layers: Repeat the process by adding another layer of graham crackers, followed by whipped cream and another layer of mixed berries. Continue layering until the dish is full, finishing with a layer of cream on top.

7. Chill: Cover the dish with plastic wrap, and chill in the refrigerator for at least 4 hours, or overnight for best results. This will allow the flavors to meld and the graham crackers to soften.

8. Serve: Once set, carefully slice the cake into portions. Garnish with extra berries if desired for an appealing presentation.