In a large mixing bowl, whip the heavy cream with an electric mixer on medium speed until it begins to thicken.
Gradually add the powdered sugar and vanilla extract, continuing to whip until stiff peaks form.
In a 9x13 inch baking dish, spread a thin layer of whipped cream on the bottom.
Place a layer of graham crackers over the whipped cream, breaking them as needed to fit.
Spread half of the remaining whipped cream over the graham crackers, then layer half of the mixed berries on top.
Repeat the layers with another set of graham crackers, the remaining whipped cream, and the rest of the mixed berries.
Cover the dish with plastic wrap and refrigerate for at least 4 hours or overnight to allow the flavors to meld and the graham crackers to soften.
Before serving, garnish with fresh mint leaves if desired.