In a large bowl, whisk together the vanilla pudding mix, milk, and vanilla extract until smooth. Refrigerate for about 15 minutes to thicken.
In a large trifle dish or a glass bowl, layer half of the pound cake cubes at the bottom.
Spoon half of the prepared vanilla pudding over the pound cake layer.
Add a layer of half of the sliced strawberries and half of the blueberries on top of the pudding.
Spread half of the whipped cream over the berry layer.
Repeat the layers with the remaining pound cake, pudding, strawberries, blueberries, and whipped cream.
Finish with a dollop of whipped cream on top and garnish with fresh mint leaves if desired.
Chill the trifle in the refrigerator for at least 1 hour before serving to allow the flavors to meld.