Preheat the oven to 350 degrees Fahrenheit. Spray a 13×9 inch baking dish with non-stick spray and set aside.
In a large bowl, whisk together the milk and chocolate instant pudding mix until well combined. Add the chocolate cake mix and stir until fully incorporated; the mixture will be thick.
Fold in the chopped mini Reese’s PB Cups until evenly distributed throughout the batter.
Pour the batter into the prepared baking dish, spreading it out evenly.
Bake for 25 minutes, or until a tester inserted into the center comes out clean.
Allow the cake to cool slightly before serving. Serve warm or at room temperature, and enjoy the rich, chocolatey goodness with bursts of peanut butter flavor in every bite!