Prepare the slow cooker with nonstick cooking spray.
In medium-sized mixing bowl add berries then cover with sugar, cornstarch, and vanilla. Toss until the berries are covered. Add berries to the bottom of the crockpot.
30 ounces frozen mixed berries, ¼ cup sugar, 1 Tablespoon cornstarch, 2 teaspoons vanilla extract
Sprinkle the cake mix evenly on top of the berries.
15 ounces vanilla cake mix
Dot the cake mix with slices of butter.
½ cup butter
Cover the slow cooker and cook on low for 4 to 5 hours.
Serve the cobbler warm with vanilla ice cream or whipped topping, if desired