Spray a 6-quart oval slow cooker with cooking spray.
Using an electric mixer, beat cake mix, water, eggs, 1/2 cup peanut butter, and butter on low speed for 30 seconds and then on medium speed for 2 minutes.
Pour batter into slow cooker. Cover and cook on HIGH for 1 hour 45 minutes to 2 hours.
(If your crock pot runs hot, check after 1 hour and 30 minutes.)
Remove slow cooker insert and place on a wire rack to cool for 15 minutes.
Make glaze. Whisk together peanut butter, milk, and powdered sugar in a medium bowl. Spread on top of cake.
Sprinkle fudge sauce and peanut butter cups on top.