Lightly spray the bottom of your Crock-Pot Express Crock with non-stick cooking spray. Add the blackberries, brown sugar, cornstarch, and lemon juice to the bottom and stir to combine. (Note: You may need to add additional water to allow pressure to get high enough to cook mixture.)
In a medium bowl, combine the melted butter and cake mix until it is crumbly, then evenly spread the cake mix mixture over the blackberries.
Place the lid on the Crock-Pot Express Crock and slide it into the locked position. Ensure that the steam release valve is in the “Seal” (closed) position. Press the Dessert button, set the pressure to HIGH and the timer for 30 minutes, and then Start. The pressure cooker will build pressure and heat for 10-15 minutes during the HEAT setting and then will turn to 30 minutes.
Once the time has elapsed and the machine beeps to signal the end of cooking time, carefully use a fork or other kitchen utensil to flip the valve to open to quickly release the pressure. Be careful because the steam that rushes out will be very hot. Do not touch the valve or the steam with your hands.
Once the steam has released, open the Crock-Pot Express Crock lid by unlocking it, then carefully remove the cooking pot using potholders and let stand for 5-10 minutes on a cooling rack so it can stiffen up and settle.
Top with ice cream and enjoy!