Mix the cake mix, water and butter in a large bowl until it is smooth. It is okay to have some lumps. Add the mini peanut butter cups.
Spray the slow cooker using non-stick spray. Spread the batter into the slow cooker.
Cover the pan and bake on HIGH for 2 hrs. Keep the lid closed during this time.
To stop the cake from burning, turn off the heat after the timer has ended.
Add the peanut butter to a small saucepan set on medium heat. Mix until smooth. Be careful as it can burn quickly. Mix in the powdered sugar. Whisk until smooth.
Spread the sweetened peanut butter on top of the warm cake. Add halved Reese peanut butter cups to top the warm cake.
Enjoy! Serve with ice cream, whipped cream or both.