Spray slow cooker generously with non-stick cooking spray.
Toss together the raspberries and blackberries in the base of the slow cooker and toss with the sugar, cinnamon and cornstarch.
In a small mixing bowl, combine the yellow cake mix, vanilla and melted butter. Toss well until crumbly mixture forms.
Spread the topping evenly over the berries in the base of the slow cooker.
Set on low and cook for 3 hours.
Serve Raspberry-Blackberry Cobbler with vanilla ice cream or whipped cream.