Spray a 6 quart crock pot with cooking spray or use a Crock Pot liner.
Using a large bowl, mix together the cake mix, water, eggs, 1/2 C peanut butter, 1/3 C butter, and 1/2 C chocolate syrup until combined
Pour batter into the crock pot and cook on high heat for 1 hour
Check to see if still doughy, if so continue to cook in increments of 15 minutes until the cake is not so doughy anymore
Remove the Crock Pot and place onto a wire rack
In medium bowl, whisk together the remaining 3 tbsp peanut butter and 3 tbsp whole milk until smooth. Gradually whisk in powdered sugar.
Spread peanut butter glaze over the cake
Sprinkle the mini Reese cups and Reese pieces
Scoop into bowls and serve with whipped cream or ice cream