6 regular or 12 mini peanut butter cups, roughly chopped
Ice cream, for serving
In a large bowl, mix together the chocolate cake mix, chocolate pudding mix, sour cream, eggs, vegetable oil, water, and vanilla. Using an electric hand mixer, mix on low speed for 30 seconds then turn the mixer up to medium and mix for two minutes.
Prepare your slow cooker by lining it with aluminum foil or parchment paper and spray generously with non-stick cooking spray. Pour the batter into the slow cooker and set aside.
In a microwave safe dish, heat up the peanut butter until it is nice and runny. Pour it over the cake batter and, using a butter knife, swirl the peanut butter through. Pop half of the roughly chopped peanut butter cups on top, cover, and cook on low for four to five hours or until the sides of the cake begin to pull away from the sides of the cooker.
Serve warm topped with ice cream and extra peanut butter cups.