Preheat your oven to 350°F (175°C) and grease a 9 x 13 inch baking dish with nonstick cooking spray.

Prepare the chocolate cake mix according to the instructions on the box.

Pour the prepared cake batter into the greased baking dish and set aside.

In a separate bowl, mix together the softened cream cheese, softened butter, creamy peanut butter, and vanilla extract until well combined.

Gradually mix in the powdered sugar, 1 cup at a time, until all 4 cups are incorporated and the mixture is smooth.

Spoon dollops of the peanut butter mixture over the chocolate cake batter in the baking dish, then lightly spread it around to create a marbled effect.

Sprinkle the halved Reese's peanut butter cups and chocolate chips evenly over the top of the cake batter.

Bake in the preheated oven for about 45-50 minutes, or until the cake is set and a toothpick inserted into the center comes out clean.

Allow the cake to cool slightly before serving.