In a large bowl mix together the cake mix, water, butter, and eggs until smooth. Some lumps are ok, that is normal. Stir in the mini peanut butter cups.
Spray the slow cooker with non stick spray. Add the batter to the slow cooker and spread out into an even layer.
Cover and cook on HIGH for 2 hours without opening the lid during the cooking time.
When the cooking time is done, remove the cake from the heat, to keep it from continuing to cook.
In a small pan set over medium heat on the stove top, add the peanut butter. Stir until melted and smooth, watch closely because it will burn quickly. Add the powdered sugar and whisk until smooth.
Pour the sweetened peanut butter over the warm cake. Top with halved Reese's peanut butter cups.
Serve and enjoy! Great with ice cream or whipped cream.
You can make Reese's Peanut Butter Cake in the oven. Bake in a 9x13 pan, and bake as directed on the cake mix box.
To store, let the cake cool on the counter and place the entire slow cooker insert in the fridge if desired. Flip the lid over to make it fit better in the fridge. Or spoon into airtight containers (this can be messy).
Reheat by heating single servings in the microwave for about 30 seconds.
Coffee - Use freshly brewed coffee in place of the water, it will have a slight hint of coffee and add depth of flavor to the cake.
White Cake Mix - Not a huge chocolate fan? Use a White cake mix instead of chocolate and you will have a wonderful vanilla and peanut butter dessert.
Reese's Pieces - Use Reese's pieces instead of cups for a fun twist! Or you can add a handful of both for some added texture.