Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking dish with cooking spray.
In a large mixing bowl, combine the peanut butter, milk, vegetable oil, eggs, and vanilla extract. Blend until smooth and creamy.
Gradually add the chocolate cake mix to the wet mixture. Stir until just combined and there are no visible lumps.
Pour the mixed batter into the prepared baking dish, spreading it evenly.
Sprinkle the semi-sweet chocolate chips and chopped Reese's Peanut Butter Cups over the top of the batter. Press them gently into the batter.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs.
Remove the cake from the oven and let it cool in the baking dish for about 10-15 minutes. Cut into squares and serve warm, optionally topped with whipped cream.
Notes
Serve warm with whipped cream and extra toppings for a delightful treat.