Start by preheating your oven to 350°F (175°C). Ensuring your oven is at the right temperature is key for even baking.

Step 2: Prepare the Baking Dish

Lightly grease a 9×13 inch baking dish with cooking spray or butter. This step helps in easy removal of the cake once baked.

Step 3: Mix the Cake Batter

In a large mixing bowl, combine the chocolate cake mix, eggs, vegetable oil, and milk. Mix well until you reach a smooth and lump-free consistency. The batter should be rich and thick.

Step 4: Add the Peanut Butter and Vanilla

Gradually incorporate the creamy peanut butter and vanilla extract into the batter. Use an electric mixer on medium speed to ensure everything is well-blended.

Step 5: Fold in the Chocolate Chips

Gently fold the chocolate chips into the batter using a spatula or wooden spoon. This ensures that the chips are evenly distributed throughout the cake.

Step 6: Assemble the Cake

Pour half of the cake batter into the prepared baking dish. Sprinkle half of the chopped peanut butter cups over the batter. Pour the remaining batter over the peanut butter cups, and finish by topping with the remaining chopped peanut butter cups.

Step 7: Bake

Place the baking dish in the preheated oven and bake for 25-30 minutes. Check for doneness by inserting a toothpick into the center of the cake; it should come out mostly clean with a few moist crumbs.

Step 8: Cool and Serve

Allow the cake to cool for about 10 minutes before serving. Optionally, dust with powdered sugar for a sweet finish.