1. Spray an oval-shaped 6 quart slow cooker with baking spray.
2. In the bowl of your stand mixer, combine the cake mix, water, eggs, butter and vanilla and mix until combined.
3. Add in the peanut butter and mix until smooth and fully incorporated.
4. Pour the batter into the prepared slow cooker and then drizzle the chocolate syrup on top and use a butter knife to swirl it through the cake.
5. Cover and cook on high for 2 hours to 2 hours and 15 minutes. Cake will be cooked through when a toothpick inserted in the center comes out clean.
6. Remove the slow cooker insert and place on a wire rack to cool. Let cool for 15 minutes.
7. Meanwhile, prepare the topping by adding the peanut butter, milk and powdered sugar to a bowl and whisking until smooth and thick. Add an additional tablespoon of milk if the mixture is too thick.
8. Use a silicone spatula to spread the peanut butter frosting on top of the cake.
9. Sprinkle the chopped Reese’s on top.