Preheat the oven to 350℉. Spray a 9x13 baking dish with nonstick cooking spray. Set aside.
In a large bowl, stir together the frozen berries, 1/2 cup of the French vanilla cake mix, brown sugar and cornstarch until well combined and the berries are coated.
2 bags (16 oz each) frozen berry medley, 1 box (13.25 oz) French vanilla cake mix, 1/4 cup packed light brown sugar, 1 tablespoon cornstarch
Spread the fruit mixture into an even layer in the prepared baking dish. If there is extra dry ingredient mixture in the bowl, sprinkle it on top of the fruit in the pan.
In the same empty mixing bowl, stir together the remaining cake mix, milk, and melted butter just until combined.
2/3 cup whole milk, 1/2 cup (1 stick) salted butter
Drop the batter over the berries and. gently spread it out as evenly as possible. There may be bumps and holes and this normal.
Bake for 45-55 minutes or until the top is golden brown, the fruit is bubbly, and a toothpick inserted into the thickest part of the topping comes out with crumbs and no wet batter.
Remove from the oven and let it cool for 5-10 minutes. Serve warm with a scoop of vanilla ice cream or whipped cream. Enjoy!