Spray your crockpot with nonstick spray.
Add the frozen berries (do not thaw), vanilla, cornstarch, and fresh lemon juice directly into the crockpot.
Mix everything together until you no longer see any white from the cornstarch and everything is fully combined.
Evenly sprinkle the dry cake mix over the top.
Do not mix.
Slice the butter into thin pats and place evenly over the cake mix, covering as much surface area as possible.
Cook on HIGH for 2 - 3 hours, or until the top is set and lightly golden.
Serve warm with whipped cream, ice cream, or Cool Whip.