Whisk together cake mix, water, melted butter, and eggs in a large bowl until smooth.
Add the 8 oz. package of mini peanut butter cups and mix in.
Lightly grease the inside of a slow cooker and pour batter into the slow cooker.
Cover and cook on HIGH for 2 hours.
To prepare the frosting, melt peanut butter in a small saucepan over medium heat.
Whisk in confectioners sugar and pour frosting over cake.
Top cake with chopped peanut butter cups and serve.
ENJOY!!