In a medium bowl, combine the sliced strawberries and blueberries. If desired, add a teaspoon of sugar to the berries to enhance their sweetness. Set aside.
In a separate bowl, mix the prepared vanilla pudding with the vanilla extract until well combined.
In a large glass trifle dish or individual cups, layer the ingredients starting with a layer of pound cake cubes at the bottom.
Add a layer of the vanilla pudding over the pound cake, followed by a layer of the mixed berries, and then a layer of whipped cream.
Repeat the layering process until all ingredients are used, finishing with a layer of whipped cream on top.
Garnish with fresh mint leaves if desired. Cover and refrigerate for at least 1 hour before serving to allow the flavors to meld.