Preheat the oven to 350 degrees fahrenheit and grease a 9x13 inch baking dish.
Place the frozen blackberries and brown sugar into the 9x13 inch pan and stir to combine. Gently separate the blackberries so that they are evenly distributed on the bottom of the pan.
Sprinkle the cake mix over the frozen berries and smooth gently with the back of a spoon.
Pour the milk and butter over the cake mix as evenly as possible. Gently tilt the pan back and forth until all of the milk and butter cover any exposed dry cake mix.
Bake for 35-40 minutes, until the top is golden brown and the berries underneath are bubbling slightly.
Allow to cool for 10-15 minutes.
Serve warm with whipped cream or ice cream, optional.
Notes
If you use frozen berries in this recipe, be sure to add them when they’re frozen. If you let them thaw, they’ll be mushy and the consistency will be off.
Let the cobbler sit for at least 10-15 minutes before serving because it will be extremely hot inside.