Preheat the oven to 350 degrees F. Spray a 9-inch square baking dish with nonstick cooking spray.
Combine the berries, sugar, cornstarch, lemon juice and vanilla in a medium bowl. Toss to coat the berries. Pour the mixture into the greased baking dish, then spread the cake mix over the top in an even layer. Scatter the butter cubes over the top. Cover the baking dish with aluminum foil and bake until the top is puffed and golden and the fruit is bubbling, about 40 minutes. Remove the foil and continue baking for 5 minutes more.
Let the cake cool slightly before scooping and serving with vanilla ice cream.
Any combination of fresh or frozen blueberries, raspberries, blackberries or strawberries will work. Hull and chop strawberries into bite-sized pieces. We used blueberries and strawberries for our cake.