Stir 2 pounds of berries, 3 tablespoons sugar, and 3 tablespoons cornstarch together in a 12-inch cast iron skillet or 9X13 baking dish, or anything ovenproof with similar dimensions.
Pour 1 (16 to 18 ounce) box of cake mix evenly over top. Scatter 12 tablespoons of butter, cut into cubes, over the top.
DO NOT MIX!
Bake at 350°F for 50 minutes to an hour, until the cake turns light brown, but still very soft looking. No worries if there are a few dry spots, they’ll dissolve right in when you scoop it out.
Serve with Vanilla Ice Cream and/or whipped cream!