In a big mixing bowl, combine the cake mix, instant pudding mix, sour cream, eggs, oil, and water. Mix until everything is well combined. Stir in the peanut butter chips.
Place peanut butter cookies at the bottom of your Crockpot™. Pour the cake batter over the cookies. Cover and cook on high for 1.5 hours.
Remove the lid. Drizzle melted chocolate over the cake and sprinkle with peanut butter cups and mini marshmallows. Cover and cook for an additional 30 minutes.
Serve it up warm, and enjoy the deliciousness!